Could you explain why Signal users are more careful than email users? Aren't all Signal users also email users?
I suppose the reverse isn't true, and there are machines that send people transactional emails (e.g. receipts for online purchases) which it would be nice to secure with PGP.
The real problem with securing email, from my perspective, is the difficulty of creating a UX which accurately and intuitively conveys to the user whether the message they are sending is secure (and what "secure" means). By using a separate app which never sends plaintext, that's basically a non-problem.