My guess is that by the time the self-selection of people who think they're ready to apply and have a good enough shot to make it worth all the time the interviews take, happens, and they get to an in-person (having perhaps passed a less-harsh phone-screen leetcode question), most further leetcoding isn't doing much aside from keeping the self-selection effect strong.

The other parts of the interview might be doing something per se useful, I suppose.