Its crypto from the 90s and email is not meant to be secure. Some examples:

Only the mail body is protected, headers and metadata arent, so an attacker still knows with who you are talking about what.

Replies contain the full thread, if one person messes up once, the entire chain is unencrypted.

No ephemeral keys, no cleanly defined rotation mechanism. If someone gets your key, all past messages are also accessible.