Arnold writes:

Think tanks, like universities, may be ripe for disintermediation.
Although I think my writing for Cato helped my personal brand, I would
rather be viewed as an independent scholar. I view scholars as personal
brands, and I do not think that affiliation with a think tank adds
anything to their personal brands.

I can think of one thing that think tanks add to personal brands: economies of scale in fund-raising.  How many independent scholars manage to support themselves on donations?  Almost none.  How many think tanks manage to support themselves on donations?  Almost all. 

Fund-raisers are selling affiliation.  And affiliating with a team of scholars is a lot more satisfying for the typical donor than affiliating with lone wolves.