I always figured leetcode was just a slightly harder version of fizzbuzz (because fizzbuzz really is a bit too easy.) You can often puzzle out a solution based on experience alone, if you've been programming long enough.

I guess the leetcode idea fell afoul of goodhart's law, and a lot of people just started memorizing algorithms instead?

Fizzbuzz is a basic competency check, it really doesn't require any "clever" thinking. I haven't looked at many leetcode exercises but the ones I have so far definitely go beyond that, and often place "unrealistic" constraints (e.g. not being able to use standard library functions). Which isn't to say they don't test anything useful, but as someone involved tech-screening hires, I doubt I'd ever bother with a leetcode exercise. Mind you I wouldn't use Fizzbuzz either, but only because it's too well known and easy to regurgitate a standard answer for.