Why Isn’t True Zero-Trust Encryption Used in Email Systems?

Most “secure email” platforms still rely on provider-managed keys:

either the provider generates the user’s key pair, or stores an encrypted copy for sync, or encrypts stored mail with server-side keys. In all cases, the provider retains theoretical decrypting capability.

A strict zero-trust model would require:

user-generated private keys

provider never storing or handling private keys

stored mail encrypted only with the user’s public key

no server-side key material that could decrypt data

provider cryptographically incapable of accessing message content

This model is common in password managers and zero-knowledge file storage, but rarely seen in email.

So the question is:

What stops email providers from adopting true zero-trust storage?

Is it:

key-management friction for users?

multi-device sync challenges?

server-side search/indexing requirements?

business reasons?

or just legacy expectations around email UX?

Curious how others in the dev/crypto community see this.