Well, for one, the integration isn't seamless. There are tons of seams, and people whine endlessly about them. "I can't find anybody taling about X,Y, or Z!" "The follow counts are different!" "Other people are seeing different replies!" "How dare you defeserate from that other website, even though it's ummoderated and its users kepe violating the rules here?"

The subscription model requires you to know about things happening elsewhere and then go out of your way to subscribe to it. ActivityPub servers are not passively finding other servers, new groups, or new people. If someone on your local server hasn't already done the "enter the url into the search bar" dance, there's nothing to see. Federation is a perk of the system, not the core feature.

For another, scaling a distributed, federated model where everyone is most active talking to people everywhere else works very poorly. Feseration works by requesting and then locally storing copies of remote posts. Every remote communication you make increases the storage and bandwith costs of your post by a factor of the number of remote servers are subscribed to it. The whole experiment breaks down amd totally destroys smaller instances if everybody jumps on the fediverse and treats it as a global-first platform, and that really kills the whole decentralization thing.

It all works best if most of our communication happens locally; if we find servers hosting the people we want to engage with the most, and join them there, and then use federation to engage with the people we're less focused on.

Treating the fediverse like modern, centralized social media, but just without the fear of billionaires, is treating screws as if they were nails. They may look similar, and they may serve similar functions, but they operate totally differently, and trying to use them in the exact same ways is going to ruin your project.

Do you consider email to be local first.

Do you consider this to be email?