> I get that they need to cut a large amount of people before live interviews

You've just answered your own question. Leetcode is meant to be a filter, nothing more.

> but hackerrank/code signal have the ability to test

I'm not familiar with those things, but I don't think companies using things like leetcode actually care how good they are at skills/knowledge testing. I think they just want a coarse filter that doesn't depend on a human making initial assessments. Leetcode does that.

And it works! For instance, I can't imagine being willing to engage in a leetcode test to get a job, so I am automatically filtered out of some positions without them doing a thing.