Final Round Virtual Onsite with DoorDash L5

1. System Design - I think I did great
2. Behavioral with hiring manager - I did not feel I did bad
3. Coding - repeated hard question
4. Coding - repeated hard question

Result: Reject!

Coding problems are very hard, even the problem description itself (that this interviewers chose) takes more than 1/2 page of standard A4 page. Reading the question and be able to understand the question could take about 10-15 mins (particularly if you haven't seen the problems before).

Why did the interviewers at DoorDash keep choosing very hard repeated problems?
If they expect candidates to excel, candidates have to do a lot of preparations in order to be successful.
I have been studying and preparing for weeks and I was able to just go straight to the optimal solution because I knew that the time will be running out and I will fail the interview if I don't come up with the optimal solution quickly. I was able to explain the non-optimal solution as well.
It is almost impossible to finish the code and explain everything to the interviewers if you don't do a lot of studying and preps.

However, what surprised me was that the interviewers seem NOT HAPPY when I was able to come with the right and optimal solution. Yes, I have seen the problems before (because these are repeated questions!) and I was able to explain the algorithm that I chose, even with my own idea to choose a better optimization than what the solutions elsewhere provided!
But it took me weeks & months to prepare and study all of these algorithms!

If the interviewers expect candidates to struggle and fail, why even interview??
Why chose repeated questions? Do these interviewers expect candidates to come up with brand new algorithms? If that's the most optimal solution, then yes those are the most optimal solutions!
What's the point here? Wasting time? Practice interview?
It could also be related with job security or any other reasons that don't have anything to do with candidates's skillsets.