You're expressing one of the most frequent frustrations people bring up with the federated model.
To my mind, it's a matter of individual user expectations. There are a lot of people for whom centralized social media became "the internet," and the idea that websites were ever intended for anything but passive external linking to individual pages is foreign. This is not a universally shared starting point.
I have a .world account for my local community and, at this point, all non-Beehaw fediverse stuff. I'm still on a few message-board sites for niche topics that Reddit's generalization made difficult to get useful information about among the detritus.
Beehaw and .world serve different needs, so I view having multiple Lemmy accounts as being as reasonable as having a different login for my natural gas company than my electricity supplier.