I agree that would be the ideal, but it's not necessarily the reality.
You address something important in pointing out that Facebook groups work this way. It was always my hope for Hive that communities would get more traction... it makes more sense to have someone join a gardening/writing/gaming community on Hive than just have them "join Hive."
People typically don't say that they are "on the Internet" or "on Facebook" anymore... it's about what they do and belong to within those contexts.
One of Hive's weak points is definitely that it doesn't have much of a mechanism to make its content of interest to people outside Hive's walls... and even when that does happen, it's a complicated and even daunting process to get an account just so you can engage with someone's content.
And so, we end up with this current situation where most are creating ON Hive for people ON Hive... no external expansion.