Ask HN: Why big email providers don't sign the email?

I completely understand the resistance of Gmail and the likes to full end-to-end email encryption.

What I don't understand is the resistance of cryptographically sign the outbound email and discard incoming spoofed emails that don't have proper signatures. This simple move would create a high barrier for phishing emails since they don't have valid signatures for the organization that supposedly sent them.

Do you have any insight in this scheme ?