Quote from flat5:
Because it isn't productive. It's a way to take money from somebody else's pocket and put it in yours, but accomplish nothing in the process, much like shuffling cards around on a table in a poker game.
People have more respect for people who earn their money by producing something in an exchange that is not zero sum.
I think that's the root of it, and it's not particularly irrational.
Increased liquidity, smaller spreads!