Why to people think that questions about job are impolite? When one works, they spend majority of their day at job. So it will define them in some way.

When I was unemployed questions like that bothered me a bit... but that was MY frustrations about MY situations not people being impolite.

As for people asking "why are you on disability" after you said you are... can be curiousity. Concern. Doesn't have to be meant rudely. If you don't want to share, don't dig in the topic in the first place.