edit: It is also very reasonable to study having done it myself and talked to people who have gotten into these companies. At 15 minutes a pop you can do 100+ leetcode questions a week. Spend a few months and you'll have studied almost every reasonable question. Most of them are repetitive patterns but around 5% have tricks you need to memorize. That covers 2-3 out of your 5 interviews. Behavioral you need to memorize around 5 questions that they'll basically give you ahead of time. System design is harder but there's videos for the 10 topics that will cover 90% of the potential questions.
By that logic, most people would have completed all 1500 LC's in 4 months.
Numbers say otherwise. Each medium difficulty problem takes an hour to solve, assuming you can come up with a non-brute-force solution. Most of them force you to lookup, understand and implement other people's ideas.
If you can do 3 problems a day, completely on your own without any lookup, you're doing good pace.
When someone comes in and bombs, that is at least valuable information: we don't want to work with this person. When someone comes in and seems smart and capable, but makes hardly any progress on the problem because it's obviously their first time on a whiteboard, it's just regrettable all around.
We're not selecting for preparedness. In fact, we try to select against it: interviewers take pride in designing twists that separate understanding from recall. If you seem to be recalling, we're supposed to pick a different question, or failing that, report a null signal. But we do need you to be prepared enough that your interview skills don't get in the way of your actual skills.
edit: I'm talking about large tech companies, not the startups blindly copying their policies with little thought or consideration.