For one thing, I always try to provide a tested answer, but doing so means that someone frequently jumps in front of me, scoring the reputation. In many of these cases I can see that the answer provided might be conceptually correct, but wasn't actually tested, because there are typos in the code. Thus, it appears that the incentives are skewed away from providing the highest-quality answers.
I pretty much quit about two weeks ago. I spent time formulating an answer [1] that was entirely correct, so by the time I posted it there were three other answers there, but they relied on special cases rather than being generally correct. One person upvoted my answer, but almost immediately, someone else downvoted it. The only thing I can imagine is that one of the "competing" answerers wanted to ensure that his mostly-correct answer was listed above my completely correct one, so he could score points from readers that hadn't yet seen my reply that explained why special care was needed.
If my diligent work is going to be devalued like that, then I don't see the point in investing more time in it.
[1] http://stackoverflow.com/questions/2975588/selecting-a-range...