Google's requirements differ. Also if the interview question was about distributed systems, "I'll just use Postgres" is not super constructive

Google has good in-house database software that fit their requirements. I wouldn't say that I can do better in an interview.

Yes but that in-house database software did not fall from sky. Google developers wrote it. Remember their interview process is generalised. Even for candidates not expected to implement databases, they test for the awareness of tradeoffs posed by distributed databases: read-after-write/eventual consistency, linearizability, how many masters /quorum/leader election, etc. "Just use Postgres" does not demonstrate any of that, it's not what the interviewer was after.

That's true. We don't know how was the interview, perhaps it was about implementing a distributed database from scratch, perhaps it was about storing as many personal data as you can.

I would hope the answer to "how would you write a distributed database from scratch" isn't "I'd use PostgreSQL."

Actually, I'd probably do the same.