Again, if this "experience" so "rudimentary", why do top engineers spend time on leetcode trying to get better for interviews? Wouldn't this "rudimentary" experience be learned through their, you know, actual work experience?

Because these "top engineers" are self-taught with massive gaps in their base CS education? Reversing a tree tests for the knowledge of binary trees and recursion, which is... what?... a second year of a CS program, conservatively?

Go on Blind. There are plenty of engineers at top companies in the Bay Area talking about “grinding leet code” as a part of their job searches.