"...What's it with humanities people that make them believe that only them, of all people, are able to do critical thinking?..."

The same thing that is with everyone else. If you look around our societies today it seems almost a matter of course that the first argument deployed in any debate is a dismissal of the opposing view holder's ability to think critically. Not to put too fine a point on it... but you can see that argument employed a lot here on HN. It's not really a matter of STEM people being dismissive of others' critical thinking skills - or even Humanities people being dismissive of others' critical thinking skills - as much as it is a matter of ALL people being dismissive of others.

It's, kind of, the nature of discourse in our society these days.