Basically, because all the lefties (and I mean the real lefties, not the 51st-percentile leftish types) need to wind up somewhere, and they don't much like work

Basically, because all the lefties (and I mean the real lefties, not the 51st-percentile leftish types) need to wind up somewhere, and they don't much like working in the private sector where they're expected to aim towards (gasp!) profit. So they wind up in society's other nooks and crannies, with the university system being one of the largest. If you're smart and left-wing you become a professor. If you're moderately smart and left-wing you become a university librarian. If you're dumb and left-wing you wind up working at the DMV. Incidentally, while science professors can actually be a reasonably ideologically-diverse bunch, I doubt you'll ever find much diversity among university librarians.