Why the constant use of uniforms?

[Christopher]

ON TOS even civilians had Uniforms!!!

Those aren't uniforms, they're work outfits. The people in that shot are farmers. Those jumpsuits are probably the equivalent of present-day overalls.

 

[Scout101]

And then there's Jerry Seinfeld's take on the issue:

I think eventually fashion won’t even exist. I think someday we’ll all wear the same thing. Because anytime I see a movie or a TV show where there are people from the future or another planet, they’re all wearing the same outfit. Somehow they all decided, “All right, that’s enough. From now on, this is going to be our outfit. One-piece silver jump suit, with a V-stripe on the chest, and boots. That’s it. We’re going to start visiting other planets and we want to look like a team.”

That's the first thing that popped into my head when I read the thread title!