>I really don't know why that happened.

Once upon a time there was a google engineer who wrote a book called the "crack the coding interview" and the rest is just layers upon layers of BS piled on top of the dogma teachings of that book, until red-and-black trees and divide-and-conquer are no different than some relics you find in a cult.

>But today why should I invest my free time into leetcode, instead of learning something useful?

If it is not useful for creating profits, then it is not useful - logic of capitalism 101.