They can, but to throw a kick a lot of times you need room to throw it. A lot of the places fights tend to happen are very cramped spaces. On top of that, real fights start in much different fashion, often with a sucker punch or a sneak attack with some kind of setup coming first. Time to actually throw a kick is a luxury in those situations.
Also, I don't like throwing kicks because they put you off balance. You slip on some beer that was spilled, you get pushed by one of the guy's friends, you had too much to drink and your balance ain't great to begin with... There's a lot of complications that come with it.
I haven't seen a kick thrown in a real fight since I was in grade school, and they didn't work then either.
I realize people are going to have different opinions and I don't want this to turn into a style vs. style thing, but boxing is simple and effective... and most importantly, it teaches you how to slip punches and hit back immediately- something that most other arts are sorely lacking in. It becomes reflexive to do these things.