It's again a user expectation problem. What if you connect to a web site for the first time while on a rogue public network?

If users expect to be "safe" when on a secure site, without them understanding intricacies of certificates, self-signed is counter productive.

There are certainly improvements to be made to the experience, but none of that can explain all these nuances in a way a temporary visitor will read and grasp.

OTOH, it's easier to teach them "HTTP unsafe, don't type anything private".