Because browsers have a UX for certs that's designed around the common/high-risk case e.g. average users going to banking websites, and deliberately terrible UX

Because browsers have a UX for certs that's designed around the common/high-risk case e.g. average users going to banking websites, and deliberately terrible UX for self-signed certs. Compare with SSH, which prompts to save the fingerprint on first connect, and warns loudly if the fingerprint changes. This is a superior way to handle self-signed certificates.