For largely the same reasons why graduating from top schools is given more credit than any old college, even if the level of teaching is similar. Getting in takes determination, intelligence, and discipline. Same with getting good at Leetcode.
This feels like a retcon. The real reason is most likely that Google started interviewing people this way and then that's the culture that developed in the industry. Once a culture develops, it's very hard to change.
It is very hard to change established processed - especially when the people doing the interviews have gone through the process.
They had to suffer, so why should the people after them get an easier ride?