Because I don’t, and pretending to feels dishonest. I’ll listen if they want to talk about it, but I’m not going to act interested, and I certainly won’t ask about it on my own. What I’m trying to figure out is whether people actually care, or if they’re just playing a social game that I’m simply not interested in.
I’m probably on the autistic spectrum, which likely explains this to some extent. But that’s not an excuse - being an asshole is perfectly compatible with autism, so before dunking on me, please realise I probably agree with your criticism.
Even if you don’t care, it’s probably a good idea to at least pay attention to the announcement of a new baby, because this is a new person, who will be a significant influence on their life and may have some impact on your own. Later on, it’s considered polite to at least ask "and how’s (name of spouse) and (name of child or children, or you can say “the kids”). One isn’t expected to remember every detail, but at least acknowledge they exist.