I think the boundary thing is the issue. My family has lack of boundaries towards me and it happens more inside families than outside, because there they dare being judgmental, nosy and bossy. Actually it's not rare to have this pattern inside families. Taking it outside is less common because usually some kind of social anxiety sets in to prevent people from making others uncomfortable.

Some are just unable to see that happening, they feel that they are entitled to that behavior. In some cases people just have crappy social skills because their neurology, but more often they are just taught this is OK. That it is OK to just spread their ego all over the place and invade others. It's a very annoying behavior. The thing is we encourage it by allowing it. If we say that it is not OK to ask that, we might do these people a favor. Judging people seem to have more social insight, they more think their opinion is right and should be pushed, but they should also be told their behavior is not acceptable.

Now I think it is a cultural thing and we don't have a lot of nosy people here. But some are and they seem to actually lack shame, totally lack social anxiety (too much is bad but a little is normal). If told it is not OK to ask what they did they can push the Why? question as forcefully. They are just very immature and act on a child's level socially.

But also there actually are no rules what you can ask about. And I'm not really happy with the existing norm that is (at least here) you ca always ask why someone is sick or on disability (Why is that normal???) but you can't ask other less sensitive stuff. It is like the more oppressive nosiness is socially acceptable. Maybe it is time we stop following norms that we believe are healthy and sane, and actually change them to what is kind and accepting.