How many of them are gonna stick around once their instance goes offline, or the admin does something crazy (which isn't impossible considering how many of these are ran as personal/fun projects by geeks rather than actual businesses), or their instance gets into a feud with the others and results in defederation?
All of this is overhead. It's overhead that can be managed, or you can pay someone to manage it for you, but it's still overhead and extra problems that just don't exist when you can instead sign up for Instagram or Twitter and call it a day.