Its rude for all the same reasons that its rude to ask about other information that is often associated with one's identity/self-worth (weight, sexual orientation, medical issues, debt, etc). Asking questions like that out of the blue will seem like you are judging the other person.
But with people you know, and in the right context, its fine to talk about such things. For example I don't mind in the slightest talking about how much I earn with my family or close friends. The same way I would be comfortable talking about my weight or sexual orientation with close friends/family.