Nothing stops Apple from offering e2ee backups, and in fact they do this for certain data backed up to iCloud (health data for example.)

But your iMessage data...well there, your ass is hanging out in the breeze. In fact, I'm not sure it's possible to log into an iPhone with your Apple ID and not have an iCloud backup immediately fire off, which means your private encryption keys hit iCloud and stay there until it is purged according to their data retention policies. And we have no idea what those policies actually are; those keys made end up stored forever.