I think (but cannot prove) that along the way, it was decided to explicitly measure ability to 'study to the test'. My theory goes that certain trendsetting companies decided that ability to 'grind at arbitrary technical thing' measures on-job adaptability. And then many other companies followed suit as a cargo cult thing.

If it were otherwise, and those trendsetting companies actually believed LeetCode tested programming ability, then why isn't LeetCode used in ongoing employee evaluation? Surely the skill of programming ability a) varies over an employee's tenure at a firm and b) is a strong predictor of employee impact over the near term. So I surmise that such companies don't believe this, and that therefore LeetCode serves some other purpose, in some semi-deliberate way.