Emails aren't encrypted, because there is no good way to manage trust between users at this point. PGP exists, but it's key management sucks in many ways. But look at a lot of large corporations where the trust can be bound to one service and encryption will be just one click away. S/MIME is a widely adopted standard (https://en.wikipedia.org/wiki/S/MIME) in email and works well.
There's no good way to manage trust between users because the services have to be interoperable. Look at messaging services like Telegram or Signal; end-to-end encryption is easy because they're single entities who decide how all messages are sent and received on their platforms.
I don't think communication services themselves have to be interpretable to manage trust correctly. See PGP used in email, git, package signing, etc. - completely unrelated actions, same trust network.
