I noticed that. It is like they are doing the guy/gal who tries to help them a favor by posting instead of the other way around. Some of these folks who answer give good researched and legit answers. I can't figure it out, why they don't acknowledge either with a positive or negative feedback.
When I ask a question, I always give helpful votes to those who take time to answer (unless their answer is truly horrible or insulting). I don't usually comment unless they say something interesting or unexpected. Or if I want them to elaborate. I guess I figured the helpful vote was a way to say thanks.
Lack of appreciation and total unawareness of good manners. If I wanted thanks I'd never bother answering. Even so, I can think of a few times I've been miffed when answers I've spent a lot of time compiling weren't acknowledged by the questioner.