Left leaning think tanks do the same.

It's, actually, a mixed bag. Brookings has an excellent reputation. I always read their stuff very, very carefully. They make mistakes, pick policy options I might not, but it's almost always done with the highest professional standards. There are others you might identify as left who would fit this bill.

My own reading is that if you take the left/right cut as some sort of basic framework, the left has been much better at fostering serious social research and living by the results. It's been Republican control of national legislative bodies (well, state bodies as well) that either reduces funding for such, suppresses the publication of such, or tries to control the results (classic illustration of this approach was the way Cheney pressured the CIA and Defense Department research staffs to produce his desired results Iraq WMD results).