The % you win is so small, you need some real money to get by. You can play Subzero's way, making a play and hoping for the line moving in your desired way, but that is a gamble in itself. Or you can have lots of outs, depending on their respective money situation, that alone can provide guarenteed profits. Like the Orioles +104, Drays +105 at two different books, automatic 4 or 5%...
Everyone doesn't do it because of the required cash, workload, etc.. But if you are a full timer with a little bit behind you, its easy money, just not a lot of it..