Or the leetcode interviews are an attempt at a second layer of screen. Looking at the school people went to may not be a great predictor of their differential success versus everyone else who went to a similar school, but it can give a good signal (on average) versus someone applying without one.

In the same way that height among professional basketball players may not be a great predictor of their performance relative to other players, but you don't see too many 5ft players eithers.