>"Colleges do. At least, liberal arts colleges do."
I'm not convinced this is actually true, having been through a Liberal Arts education myself. During my orientation the professors would proudly extol the virtues of University and how we were about to embark on a four year journey that would make us into true critical thinkers, members of a society able appreciate the world by exposing us to as many disparate subjects as possible.
In practice, though, there were a set of GenEd courses I had to fulfill in order to graduate. Of course the GenEd courses touted themselves as critical thinking crucibles but I just found them to be mere extensions of what I had been learning in High School. Rhetoric 101 and 102 wasn't about persuasion, it was about reading a common book and writing an essay about it's themes - exactly the kind of thing I had been doing since middle school. Essays were just essays. There was nothing particularly new or more difficult about the lessons designed to teach us critical thinking.
Edit:
The essence of what I am getting at is twofold. One, critical thinking IS being taught in schools already. I suspect "we don't teach kids critical thinking" is a reflexive go-to trope to explain why so many people seem irrational. But teaching these concepts is not a new revelation and teachers have been doing so for decades, if not generations. Even in antiquity people tried to teach about the difference between Ethos, Pathos, and Logos.
Two, "critical thinking" in College/University Liberal Arts curricula is not radically different from the methods used in high school. Namely, common assignments and asking the students to elaborate on their views and defend them. There is no secret method to producing critical thinkers that is accessible through college education. I assert it is merely more of the same.