Why do some people feel it's okay to ask others to switch seats on a flight, and how should you handle it if someone refuses rudely?

Some people carry too much entitlement.
Everyone has a right to refuse, it isn't rude- what is rude is to expect someone to bow to your will.
Some seats cost more. Some airlines have a fee which allows you to choose ahead of time. I'd probably say no if someone asked me, and it might even come out annoyed which could be interpreted as rude, but they should accept my "no" boundary.