A few different things going on here, but the bottom line is yes, there is a screaming opportunity for making money on easy arbitrage. You can easily and quickly make a decent living if you don't mind putting in the hours, and you can with hard work, dedication and hopefully some talent get vastly more than that.

Why don't more people do it? There are a lot of places people turn away, from the legal issues to the morality of the activity, to the low status, to the swings and need for psychological stability, to the lack of a long term plan, to the fact that poker gets boring after a while and it happens quicker if you're maximizing alpha. It also doesn't scale that well; to get past a certain point and earn top dollar you need expert-style training and talent. Marginal improvements in skill are huge over the long term in gambling.

However, I recommend that anyone who can't otherwise get a good job seriously consider poker. Gambling in general involves huge amounts of money being effectively given away, resulting in lots of low hanging fruit. Sports betting is actually even softer than poker.