It is a matter of responsibility. If you can log into any lemmy instance or mastodon server with the same account, then which server takes responsibility for your actions in the fediverse?
I have seen instances be defederate from because of their lax account creation requirements, or because of harrasment from users from a specific instance.
If an account can log into any instance, then who is responsible for banning the account?
I doubt they do mich different than you do with their OS.
People are more motivated by feelings than actual logic. The person you are responding to even states that Ubuntu “feels good to use”. That is some car advertisement level of feeling based reasoning.
Another thing is that people really hate it when things change. Especially a UI change. Every change in the Windows UI has been met with disgust. And if there is one thing different between linux distros, it’s where they place all the buttons, menus, etc. So people prefer to stay with the distro they know.