That’s just because they haven’t been taught about it yet. Once it catches on more (Twitter and Reddit refugees, Meta app) it’ll become more widely understood and more people will start using it. Once you understand the point of the Fediverse, using it isn’t a whole lot harder than any other social media.
They don’t really HAVE to think about the various instances imo. They just need to join one, that’s it. Following users/communities from other instances isn’t hard to wrap your head around, you just follow them. badda-bing badda-boom. The @instance.whatever bit of their username barely matters. You just say “that’s like a URL to find that user on a different instance than the one you chose”. People arent as stupid as you might think, they just need someone patient enough to explain.