If you are on Stack Overflow to make money, you probably are here for the wrong reasons.
I think most answerers
- want to give back since they got help before,
- want to help others, or "make the Internet a better place",
- want to build reputation (which might later help to make money [I got (some days after this answer) a job by Stackoverflow Careers, showing my reputation and some of my best answers], but doesn't have to), and/or
- simply enjoy the gaming part.
There might be some people who are paid to give answers on Stack Overflow (or on other SE sites), most probably because these are questions about a product they have to support.
Some examples: I've seen Versant employees on Stack Overflow answering questions on db4o (though I'm not sure if they are doing this in their free time or on the job), and I heard there are some Trello developers on WebApps Stack Exchange answering (and even asking) Trello questions.
But this is a small minority of all answerers, I suppose. (I don't count answers on the meta sites by SE employees, which also fall in this category.)
There are also developers of Open Source programs/libraries/packages answering questions about these (quite some on TeX - LaTeX Stack Exchange), though these in most cases don't get paid.