The warning is designed to let people know that who you're talking to can't be proven, which is important when someone tries to impersonate a bank, or your email provider, or any other number of important sites.
If we encourage users to blindly accept self-signed certificates (giving us end-to-end encryption but sacrificing identification), nothing would stop those actors from altering your HTTPS sessions as easily as they alter your HTTP sessions today. It's throwing the baby out with the bathwater.
I'm really having trouble figuring out the attack scenario unique to self-signed certificates that you don't have with plain http.