Because it doesn't come with instructions and there's a lot of "buts". For example, it took me a while to understand that, for example, I can't log in "Mastodon.world" with my "Mastodon.social" account, I had to go to "Mastodon.social" to log in and THEN I can access to the content in "Mastodon.social". Also, there's the problem of instances blocking other instances. I would be pretty salty if I couldn't access an account I followed, because the admins of the instance in which the account is hosted, decided to block the instance in which my account is in. That's why the mayority of users go to the biggest instance (Lemmy.world and Mastodon.social for example), because instances blocking the biggest instance is unlikely to happen (you would block a lot of potencial users and therefore, diminished attractiveness of your instance). But everyone doing that defeats the whole point of federalization. Also, there's the problem of defederalizing of instances. Also, the problem of safety, privacy and security. For example, a massive security fault was recently discovered in Mastodon and a server was raised by the police and all of it's data captured by the police. In simple words, the fediverse is not the panacea it was sold like, the kinks are not ironed out.
Because it doesn't come with instructions and there's a lot of "buts". For example, it took me a while to understand that, for example, I can't log in "Mastodon.world" with my "Mastodon.social" account, I had to go to "Mastodon.social" to log in and THEN I can access to the content in "Mastodon.social". Also, there's the problem of instances blocking other instances. I would be pretty salty if I couldn't access an account I followed, because the admins of the instance in which the account is hosted, decided to block the instance in which my account is in. That's why the mayority of users go to the biggest instance (Lemmy.world and Mastodon.social for example), because instances blocking the biggest instance is unlikely to happen (you would block a lot of potencial users and therefore, diminished attractiveness of your instance). But everyone doing that defeats the whole point of federalization. Also, there's the problem of defederalizing of instances. Also, the problem of safety, privacy and security. For example, a massive security fault was recently discovered in Mastodon and a server was raised by the police and all of it's data captured by the police. In simple words, the fediverse is not the panacea it was sold like, the kinks are not ironed out.
Expecting your login to work on 2 completely different domains is like… a 90s internet question.