Because encountering a site with a self-signed certificate likely means that the connection is MITM'd. In any case, with a self-signed cert you can't who you're connecting to.
CA-signed certificates do not mean that you are not being MITM'd. They are actually worse in that they give everyone (including the browser) a false sense of security that you have to pay for.
See detailed reply here: https://news.ycombinator.com/item?id=7826443