The point isnt to give a simple answer, even if it's a correct answer. The point is show how much you know and how smart you are.

The question is framed to you as a way for you to show you know x, y and z and talk about x, y and z.

Even if a valid solution is just do a, that's great. But the interviewer has no idea if you actually know about x ,y and z do they ?

The point is the interviewers are sometimes obtuse.

Sometimes the point of the interview is to see if the candidate knows an existing solution and "just use postgres" is the good answer. Sometimes it's to test the technical chops and pretending postgres doesn't exist is the point.

The candidate shouldn't be expected to always guess right, unless the position says "a psychic". The interviewer should notice if the candidate is solving the wrong kind of problem and nudge them in the right direction instead of punishing the candidate for the wrong guess.

In an interview you need to explain your thought process and demonstrate that you’re making an informed decision with supporting evidence.

Saying “just use Postgres” and then providing no explanation for why you believe Postgres is sufficient for the job, a general explanation of what type of hardware and architecture you’d use, and other details is not answering the question.