I'm guilty of not signing my emails despite associating some accounts with my key. A fear I have is signing but the receiver, even someone in the general tech field, not understanding the surrounding text/attachments and dismissing my mail. I wonder if that's part of why these large orgs don't sign.

I do agree with other comments that if big clients like Gmail abstracted PGP signing nicely for their users, it would put more pressure for organizations to publish public keys and sign lest the people they try to reach complain that Gmail gives them a nice big warning each time they try to open one of their emails.