There's a huge market for them. They're called private group chats, and they're popular everywhere. The problem is that you can't make much money off of that, precisely because they're socially valuable (i.e., they're strictly socially-driven, so there's no opportunity for in-line advertising, or recommending "new connections").
The problem I most often experience with group chats is that not everyone's on the same platform (excepting SMS) but old fashioned social media has that issue too. They also don't scale super well to large clubs or organizations, where I think having topic-oriented posts or threads as the default becomes valuable.
I consider the inability to do this a feature, not a bug. Group chats are socially segmented, with friction between those segments. That crazy uncle who loves conspiracy theories will be confined to just the group chats you have in common. You don't get insufferable loudmouths at scale, amplified by social media reward structures and algorithms.