It's really hard to exaggerate how many how silly people are playing poker. Right now I'm logged onto a pokersite that currently has 381793 people playing.

Most of those don't bother to put work into learning how to play, so the "arbitrage" opportunities indeed are huge.

Why then more people don't become poker professionals... well, I know quite a few who have, but I for one consider poker a really boring game. I couldn't imagine playing it if the opportunities weren't so good. Also, it can get really stressful if it's your sole source of income, as in many forms of poker there are strings of bad luck where you lose money. Then you can get scared/nervous, and "scared money is losing money" is a saying that's usually very true in poker.