What you are trusting, when you trust a CA, is that it will only issue a certificate for a domain to someone it has verified as the owner. A self-signed cert could be from a man in the middle attacker.

An active MITM is a relatively more exotic threat than a passive observer. In the days when certificates cost money, there was a legitimate argument that you shouldn't need the whole elaborate active-MITM defense just to get protection against passive snooping. But now that you can get both for free... just use Let's Encrypt.