Because there is no real demand, barely any benefit, but many problems it would bring. First it would need a new standard which decouples the domain-part from

Because there is no real demand, barely any benefit, but many problems it would bring. First it would need a new standard which decouples the domain-part from it's DNS and allow senders to find their target-server independent of the domain-part. Which means, there has to be someone who maintains a list of all valid decoupled mail-addresses. This would be a goldmine for spam. And you would need to adapt all software using mail, to support it. This could take a decade or longer to execute. Second, this solution has to work even when the domain itself doesn't exist anymore, has to be synced with the official mail-server, which also means any new domain-owners mail-servers. You basically wouldn't have full control over your own systems anymore on the user-part, and have an additional external liability which could harm you service. And third: Domains and usernames are valuable property, unlike phone numbers. There is serious money in this game, and regular lawsuits over who has the right to own a name. And this would poison the system and open door for abuse and even more lawsuits. At the end, the little benefit is uncomparable to the problems it would bring.