Some people are jerks. Some people are too frazzled to remember good manners. Some people were raised by wolves and aren't even familiar with the concept of manners.

I don't expect people to say thank you when I answer a question. I used to, but that expectation was beaten out of me a long time ago. But I definitely appreciate it — it's a nice surprise — when someone does bother to say thank you, and I try to remember to always say thank you myself.