LeetCode is popular, not because it works. Nah, it's for another reason.

It makes the interviewer feel good.

There's comfort in having a "objective" measure. A binary pass/fail or even a grade. It reduces the anxiety of making a hiring decision, makes it seem closer to "fair".

We choose the metric that's easiest to measure, not the one that's most predictive of success. Then we convince ourselves it's the right metric because it's the one we have data on.