You'd be shocked at how bad the average software developer is at writing software. I've interviewed many people who can't solve FizzBuzz level problems. In interviews I have people program a function to rearrange characters in a string, appropriately named "SimpleProblem", and >50% of candidates with experience can't write a function that compiles and solves the problem.

LeetCode is just what you get when you ask Software Engineers to solve the interview problem - they like showing off their chops so that's what you get. It doesn't need to be that complicated though. I know with about 80% certainty whether someone will work out based on how they solve that "SimpleProblem". The other 20% - just fire them if they don't work out. It doesn't need to be that hard. You don't need 100% certainty for a hire, you'll never get there. Just fire the ones that don't work out. You don't even need to feel bad for them - in this market they'll find another job by the end of the month.