I think it's simply because in most people's minds it goes without saying that a couple is going to have kids.
To be honest, as much as I wish to remain childfree myself, I don't think it's that weird when people tell others they're trying for a baby. Like reproducing is a pretty normal, natural and common part of a human life and having kids is a big change in one's life. Is it that weird to tell people around you that that big change in your life is most likely coming soon? I personally don't think so.
On one hand you say you find it weird that people ask about someone else's family planning unprompted but at the same time you also say you find it weird that people ask about trying for a baby unprompted. If you can't ask about it unprompted and you can't talk about it unprompted, that would make talking about family planning and kids impossible.
I realize this is going to be an unpopular opinion in this thread, but yeah I don't think it's that weird to talk about something so completely normal in a human life.