Costumes in movies serve a different purpose than the costumes we build.
Costumes in movies often don't survive in tact because the actors in them are moving around and doing what needs to be done for a shot. So the props and costumes get damaged or broken a lot. Costumes in movies only need to last for however long they're filmed. As a result, they aren't used or worn for extended periods of time like ours are. If they are shooting 3 minutes of footage for a particular cut, then the actor probably isn't wearing the suit for hours and hours on end. When you see 10 minutes of footage, it's been shot over several hours at least, and the actors take breaks and at least partially de-suit in some instances like this.
OUR costumes are an entirely different beast. We build them for longevity and for comfort. A good number of Halo costumes you see around here don't include a proper/accurate undersuit (be that for lack of necessary skills, time, money, or even just to improve comfort). For a movie,
every single aspect of a costume has to suspend disbelief. So that includes putting details where most of us wouldn't bother putting them. You may not see 50% of the details, and some details may not even be in any of the film, but if they aren't there, you'd miss them.
In addition to all that, the foam is necessary for stunt purposes. If you watch the short film, the actor gets flipped around and lands pretty hard. No stunt actor in his or her right mind would take that in fiberglass. So they make the whole thing foam to reduce the possibility of injury. Also, the actor was running pretty hard too. For the amount of detail on the suit that had to be preserved during shooting (remember, suspension of disbelief), there would be no way the actor could perform that way with fiberglass castings or even vacuum-formed pieces without substantial engineering prowess. So in order to have all the details and to allow the actor to perform the way he/she needs to perform, the foam castings are the best choice. They allow flexibility because they give, and the actor doesn't have to worry about damaging it while they perform.
So try not to think of it in convention / party costuming terms. Movie costuming is pretty different from what we do. The fundamentals are the same, but the execution and purpose behind it make it a different beast. Of course the actors are going to get hot and sweaty, but rest assured they aren't trapped in it for hours and hours.