It's not a hard concept, but it is a stupid one. Phone numbers for example you can take with you when you switch providers. Email has the provider hardcoded into your email address, thus every new provider means you have a new email.
The Web still lacks a first-class concept of a user identify that you can take with you across servers.
It's funny you mention that because for a while the supposed deal breaker with Mastodon was the inability to migrate accounts (even though no other service let you do that).
But now you can, so we're on to the next whack-a-mole.
But if you own your own domain and use it for your own mail, you do take your identity with you? Isn't this what AT protocol is promising with Bluesky's identity system?
The Web still lacks a first-class concept of a user identify that you can take with you across servers.