They genuinely believe the dumb shit they say.

It is unlikely that anyone believes what they say, as why would you waste time talking about something you believe in? You already believe it. Your mind has been made. There is absolutely nothing more that can be said. Discussion is only interesting when you can learn something new, which means exploring the things you don't (yet) believe in.

There is good reason why every comment isn't: "Hay guys, did you know that 1+1=2!?!?" – something you no doubt believe in. What are you going to learn by bringing it up again and again? Not much. There is no value in exploring what you believe.

When someone says something, they are seeking more information to validate an idea they have. With sufficient validation the idea brought forward may transform into a belief, but that transition requires exploration. Talk is how we explore ideas.

If you watch people, the pattern becomes quite apparent: Once something becomes a belief, interest is lost, and the talk about that subject will fade. They move on to something new that they want to find validation in. Sometimes people will get stuck on an idea that isn't believable. This is where you find conspiracy theorists and the like, where people build up a whole persona around an idea that they keep failing to validate.