Sure, but an embedded database is still simpler than client/server. For many tasks, postgres does not offer any meaningful benefit over sqlite so why add the complexity?
I've never worked on such a project that didn't benefit from postgres, but if I did, I would still use postgres because upgrading from sqlite to postgres, and learning a second SQL dialect, and converting a SQL between dialects sounds like a useless nightmare that costs me nothing to avoid.
I feel it's modern hype on lonely/indie/solo developer creating new thing which changes the world. Not caring on "complex" things till the project reaches 1 request per minute and finally have some data to be lost, is totally fine with this goal/scale.
Modern, production-grade, web-scale machines are able to run more than one process, these days.