Being polite in answering questions is pro-social.

It's a more pleasant interaction when someone is polite. -- Whereas, if you get rude answers, you'll be discouraged from participating.

Asking good questions is pro-social. People are going to be more willing to help if the questions are well thought out. -- Whereas, asking in an anti-social way discourages people from helping out.

It's still possible to ask questions even if you get snarky responses back; and it's still possible to answer lazy questions... But in either case, it's easy to see why people might not like that.