Off the cuff, 3 reasons. 1) Most of the prominent left wing academicians are pretty old by now, and may in a way be just the left over of the older academic system, where knowledge still counted for something. 2) If you are a young untenured academic, you'll get radicalized quickly when you see how the academia works. 3) What they write is quite impactless, so it may be safer to keep these radicals writing their papers rather than them going and doing something elsewhere. Big think tanks will pay for you to write anti-China drivel, on the other hand, and scream at every corner that "we need more bombs."