It's actually not. It has been shown that you can get extremely good Leetcode ratings by systematically learning their 15-20 most used patterns. This can be done easily in a sort time span. Guides how to achieve this can be found on Reddit.

Same happens to students if they start to memorize old training exams systematically. They exploit that professors and their assistants will recycle/adapt a decent amount of old questions. Doing this is cheap in terms of effort compared trying to understanding everything.

Of course people that succeed in LC tests will defend it. Gatekeeping is a natural response.

That's reason why we stopped using it and spent much effort in eliminating any possible preparability. You get highly qualified pattern matching monkeys if you offer them the stuff they are trained for.

But 10-15 years experience has to count for something. Why do senior and principle positions still need to leet code interviews?

Because there are a lot of fraudsters.

With social skills and salesmanship, you can never have delivered top-class work even after 15 years as a developer.

Quite a few people know how to "play the game". That's pretty easy if your superiors have no idea about software development themselves.