Another thing is also that email itself was not designed with encryption in mind and because it is decentralized and backwards-compatible it is hard to enforce new rules (everyone should use PGP).
But PGP is useful if both sender and receiver agree to use PGP.
> But PGP is useful if both sender and receiver agree to use PGP.
Anecdotally: this works until you add a third person who responds to the thread in plain text, with everything threaded beneath. Or when you do that by accident.
Applied cryptographers will tell you to use Signal or any other E2EE chat.