I don't think many people realize how intelligent and quick witted you have to be in order to be a successful hitman. It would be a waste if that type of intelligence was wasted being a contract killer.
Secondly, as already mentioned, the pay isn't worth the risk unless it's a high profile target.... which will increase the risk of getting caught.
Third, every job has operational expenses and you might want to hire a liaison to pick up and deliver your dossier for a fee so you never have to come into contact with the person hiring you.
Next, you have to be a cold blooded psychopath to kill without an emotional response mechanism.
Lastly, if you start making real money, you need to know how to receive it and clean it without getting caught. As soon as you deposit unusual amounts of money on a consistent basis that exceeds your income paper trail or someone wires you that absurd contract fee without a paper trail, that shyt will be flagged and then, you'll be on the alphabet boys watch list.
Having said all this, if I wasn't so morally and ethically grounded, I'd do it!