Because ideas with aliases and other shared key protocols require you to trust the counterparty you are talking with. They can leak all your conversation at any point without leaking their private key. There were many proposals, but they all require some level of trust in the counterparty or in a private relay operator, for instance.

This one does not. You can talk to your worse enemy and they won't be able to expose you unless they expose themselves as well (leak their main account's private keys).