I hate to dampen everybody’s grouch session — actually I doubt it will, he said sotto voce — but a lot of these are down to the facts that TV and movie dramas are as stagey as any stage play and real_ism_ isn’t real_ity_.
As was observed above, soldiers lose their bucket helmets in dramas so that one can watch the very expensive star actor do that thing they went to acting school to learn how to do in the way they were trained to do it by making their face muscles simulate expressions commonly associated with the emotions the movie makers want their audience to vicariously experience. Hiding their faces inside a helmet de-humanises them, puts a barrier between them and the audience. Since few actors are trained to act with masks or get much chance to practice it in their work, very few have any idea how to do it, and very few film-makers know how to bring out character without doing the face thing.
Computers beep, keyboards rattle, swords go shing (even when drawn from leather or lined scabbards), guns rattle like loose collections of metal components flying in close formation, spaceships roar, so that the audience will keep their attention on the screen and not be distracted by the guy crunching his popcorn three rows behind them or their neighbour singing in the kitchen or what-have-you. They do it so the audience knows what’s happening, just like the helpful actor in the stage play puts his hand to cover his mouth from the other characters and turns his head to tell the audience what’s going on. It isn’t like real_ity_ (real_ism_ is a bunch of tropes that the audience agrees represent reality, and as such varies with the audience, and should never be confused with reality, but often is), it’s theatrical device, used to tell the story, which is the point of the exercise.
The laser travels slower than light, slower than tracer bullets even, so that the audience can plainly see where the laser blast is coming from and where it is going so that they’ll understand why the bulkhead behind the hero suddenly exploded in flashy pyrotechnics. Gun bullets are allowed to be invisible because we all know about guns and how they work (or think we do), while blasters need some visual explanation — or so the movie-makers think.
The general wears his dress uniform in the field, with all his decorations and brass and scrambled egg, so that the audience knows that he’s a general as soon as he appears without having to waste any dialog introducing him. It’s not right, in fact it’s a dumb idea through and through, if the screen was a window on reality, but it’s not, it’s a window on story. These things are all things that are there to help tell the story, and they are every bit as contrived as the granny by the fire saying, “Once upon a time…” or the sound effects guy banging a metal sheet offstage to make a sound almost completely unlike thunder, or Scooby-Doo and Shaggy running by the same loop of scenery for a half-minute conversation while they discuss what’s making them run away.
It’s all contrived, none of it is real. Whether it’s realistic is up to whatever accommodation the audience makes with the storytellers.