> There is no meaningful data that any hiring process--good or bad--improves the outcome of a hire.

I don't have stats but anecdotally, hiring by personal reference seems by far the most effective way to hire.

> Given this, why not hire lightly and fire lightly?

Because it's a huge financial and organisational burden on the hirer and an even huger (sometimes existential if you live in the U.S. due to health insurance but that's a whole nother rant) burden on the hiree.