Phone numbers are just sequential numbers, that are assigned to carriers, when consumers transfer numbers the carriers just re-assign ownership via an assignment swap.

An email address by contrast is an undefined/minimally defined identifier in front of an @ and the message is just thrown to the domain to do something with.

Which is to say, mobility is impossible, because there's no cross-vendor standard for the identifier part and even if there was, both parts make up an email address which would send it to the wrong vendor anyway.

It is like having a phone number that ended with a network specific postfix and using it to route at the network level. It would make phone numbers non-transferable too.