But the most likely life experiences to do that are ones that put a person in touch with new ideas and new situations. Universities are much better positioned to generate such experiences than, say, most jobs. To some degree, those that have attempted to be at least nominally more diverse (economically/racially/...) are also the sorts of places where students are more likely to meet other people who are not like them in some important ways, and this has always been the sort of experience that preferentially tilts most people towards liberal/progressive ideas.

I believe students are much more homogenic than you find in school (eg dumb people are around) or in joining the military (you meet conservative people).