Since forever, sharding, distributing postgres / mysql was not easy. There were a few proprietary extensions. Nowadays it's more accessible.

This was typical crap you had to say to pass fang style interview "oh of course I'd use mongo because this use case doesn't have relations and because it's easy to scale", while you know postgres will give you way less problems and allow you to make charts and analytics in 30m when finance comes around.

I made the mistake of picking mongo for my own startup, because of propaganda coming from interviewing materials and I regretted it for the entire duration of the company.