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."

because leftism is so correct its literally impossible to study the world without cedeing a ton of ground

but as yanqui said they don't give them power so its quite safe for capitalism to let them analyse and occassionally come up with ideas can help the ruling class

Most academics that are left wing ostensibly (that is part of their public identity) are essentially harmless enough that their benefits (research, influence) to those in power are more valuable than the potential damage. Additionally, large swaths of the american public view academics with mistrust. In this way capitalists are able to eat the cake of progressing science without having to deal with the (to them) downside of powerful academics (who are usually left at minimum).