The snobbish answer but quite true for most people is "Because we can afford it".
I don't really get what's so snobbish about that. I mean, if I could afford it I'd have a something really nice, and when I get out of college and get a real job one of the first things I'll do is buy myself a bike I can only dream of now. If a guy has a lot of money, what do we expect him to do? Sit on it his whole life? As long as rich people with nice bikes (or poor people with nice bikes, for that matter) aren't making fun of my bike, then I won't think of them as snobs.
Anyway, wouldn't buying the best, most expensive bike you could buy be sort of anti-climactic? Part of the fun is admiring those beautiful machines that you just can't afford. Sometimes dreaming is more fun than doing. My bike is Al, has 105, and cost me $700. When I first got it I thought it was the best thing in the world. Whenever it's not the best thing in the world anymore, at least I have another level to go to. If you have the best bike out there and it's not so new anymore, what do you dream of? What do you keep working for? Maybe being poor is better than we make it out to be....