Small communities that are similar should think about mushing together for activity. Example: Hiking, Backpacking, Thru Hiking. It will generate more Activity and then branch out. We have too many niche pages that won't be active.
I see your point, and I agree that it would be nicer to just have to follow one community instead of a million smaller ones. There's always cross-posting. When you make a post and there are several communities for the same thing, post on the smallest one and then cross post it on the other ones.
Each community has a reason to exist, maybe different rules and different moderators. It's up to them to arrange a merge or the admins to archive inactive communities.