First, suits have different functions, like any clothes. As an example, you wouldn't wear football pads swimming, or a SCUBA wet suit hiking in the desert. Because spacesuits are highly engineered structures, they are designed to do there primary function as best as possible.
First, for inside the spacecraft for emergency protection you have an IVA suit. (Intra-Vehicular Activity). Then for outside the spacecraft for spacewalks or planetary walks an EVA suit (Extra-Vehicular Activity). Mobility is the key for either of these. However, in an IVA suit, the engineer is more concerned about reducing bulk and increasing comfort of the suit in an unpressurized sate. Because, unless there is an emergency loss of pressure (Bad thing), the IVA suit will never be pressurized, so you want it to be comfortable when not inflated. An EVA suit on the other hand, is always used pressurized, so it needs to have the best mobility possible. But getting good mobility is very hard to do. Once a suit is pressurized to that minimum required pressure, it turns into a rigid structure. Think of a volleyball un-inflated, then inflated. A huge difference in stiffness. (Volleyballs are inflated very close to minimum suit pressure)
So, how do you make a pressurized balloon around a human body, that, with life sustaining pressure, allows the occupant to move? According to the movies, any fabric bag will do, as long as you add some plumbing fittings on the front, a neck ring and a helmet.
The biggest difference, is Hollywood (in most cases) doesn't care to show a suit as it would look pressurized. So the suit can be baggy, and have all sorts of "cool" looking things on it, that in a real suit, would not be practical or allow the suit to function. The pressuried bit is the main function of a "pressure suit" so you would think you would think that would be important in a design. But since most people have no clue how a suit works, they are blissfully ignorant of this fact.
If you look back a "old" movies, the spacesuits look pretty ridiculous.
It seems that movie spacesuit designs have improved (Read more believable) in recent years, but have they? Many of the suits used in 1950's Sci-Fi movies where based on the real suits of the time.
Here is a "Tomato worm" suit of the 1950's (left) and the suits from the 1950 movie "Destination Moon"
Or from the 1960's the ILC SPD-143 Apollo suit (International Latex corporation) and Major Matt Mason's spacesuit
More recent spacesuit designs in movies want to be slimmer, less bulky, so they copy Mechanical pressure suits (MPC). This design tries to wrap the body with a tight layer of stretch fabric to apply the appropriate amount of pressure to the body.
A suit mock up from MIT (left) and the suit from "The Martian" (Right)
While the idea sounds great "Just slip on the "spandex" body suit and go into space". It isn't very practical. To apply the amount of pressure to the body to keep your blood from boiling, the suit needs to be very tight. In fact, even the tightest Spandex suit wouldn't work. You would die. The MIT suit tries to do this with lines of force across the body in directions that don't change length, but after 15 years of "development" they don't have a working suit, just this mock up. Could it work? Perhaps, MPC suits have been around since the 50's. But they have too many "side effects". Such as, the fact that you are covered with hair and each follicle of hair originates from a "pit" on the surface of your skin. The MPC suit can't apply pressure to these small pits in you skin, so when exposed to a vacuum, the blood pools in these areas giving you a "Hicky" everywhere there is an area of the skin not pressing against the suit. In addition, you have too make sure the suit pressuer on the skinn is very even, or you get "hot spots" on your skin. Like the elastic band of you tighty whities digging into your waist after a long day. So while this is a "cool" idea, it is not as practical as a gas pressure suit.
So while it seems movie spacesuit designs have gotten better, I think the general public's education and familiarity with spacesuits has improved. I mean we have now had people in space for nearly 50 years and continuously for the last 16, so movie costume designers have to make more detailed and complex suit designs these days to make them seem more realistic. But they still have the same design issues, and most movie suit could not be built to really work. But it's a movie, who cares. In most cases, I'll watch a movie and not care about such things. However, other time it bugs me. In recent years the Studios advertise space movies as superior in there accuracy and promote the fact that scientists and experts have been hired to insure the realism of the film. Perhaps...but no one seems to have hired people that know how real spacesuit have to function.
I mean, how would you hook up the hoses on these helmets from "Interstellar", and still get the helmet on you head?
Also when the suit pressurizes these hoses get stiff and it would be very difficult to turn you head. Why would you add hoses to the back of the helmet anyway? Also the neck area would inflate and the helmet would be pushed up over you head. But I'm nit picking details here. There are however many issues with the suits design and mobility, but you have to really know how suits work and the limits of materials and physics.
BTW the old excuse "But, in the future we will have new materials that....." doesn't hold up. In 60 years of pressure suit design, no material improvement has radically altered the way a pressure suit can to be made. In fact there are very few examples of a material improvement completely changing how we build something to work around physics.
So whats the issue?
A pressurized cloth suit is deceivingly complex. When fabricating a suit from fabric, you have to think of fabric as a bunch of "Cables" that just happen to be woven together. This is because when sewing the suit together, each thread of the fabric has to be tensioned evenly and correctly or your suit, when pressurized will warp and twist into a pretzel. Also, when you pressurize the suit, all these "cables" of your "non-streach" fabric tighten up and the whole garment grows. In fact, it grows a lot.
Here is Orbital Outfitters IS3 IVA suit unpressurized (left) pressurized (right)
The bent over look of the suit under pressure is because the suit is designed to be in a seat and this bend at the waist helps keep the suit from trying to straighten out and push you out of your seat.
If I was to make a cube out of very stiff fabric and pressurize it, it would turn into a ball with nubs where the corners were. Another issue with a suit, is mosy people are not round, we are more oval shaped. When yo make an oval suit and add air pressure, it goes round. Making it difficult to bend these now round joints.
Here is an Apollo suit without it's cover layer. (The white outermost layer, that nevr comes under tension) You can see all of the cables, bearings, rubber joints. These are what makes this suit work. Without them, the suit would be an un-bendable fabric balloon.
Even with all these specialized joints, you could not bend over and touch the ground.
Why can't you bend easy? As you bend your joints, the volume of the pressure suit changes. As the volume changes, the pressure changes. the greater this volume change, the bigger the pressure increase So, bend you elbow and the volume decreases which causes the suit pressure to rise and this increase of pressure, wants to force the arm back down to the largest possible volume again. The elbow and knee problems are basic and there are many successful solutions, hip joints and shoulder joints on the other hand, which have to move in many directions are the big problem. Bend over at the hips and there is a very large volume change, and the suit wants to spring back.
Here is the IS3 suit in a kneeling position, a bit of a "hat trick" for an all fabric suit.
This is why there are spacesuits made from hard materials. These designs increase the range and ease of mobility greatly by using mechanical design to maintain a constant volume.
I'm not a good enough writer to explain all of the issues (Nor are you probably wanting to read that much) So just be assured, there are so many things in suit design that are unseen until you have worked with them. Every solution you may think of, has either been tried or has 3 obvious (to a suit engineer) problems that keep that idea from working. Think of spacesuit design, as deep and involved as the game of chess, but you are only given half as many pieces to come up with a wining strategy.
Too a mechanical engineer and "Problem solving junkie" spacesuit design is irresistible.
I'll try to continue this with why these suits have all this hardware, as well as a look at other suit designs...