Is there a reason registrars aren't cryptographically signing these messages? That would give everyone a relatively simple way to verify that they aren't forge

Is there a reason registrars aren't cryptographically signing these messages? That would give everyone a relatively simple way to verify that they aren't forged. I get that not everyone who will receive these messages are technical enough to figure out PGP or S/MIME, but it would be trivial for Gmail, Outlook.com, Yahoo Mail, etc. to put a pretty seal on these signed messages.