Why use Polars over just postgresql or sqlite?

It is optimized for analytic workloads (default in-mem columnar layout), whereas PG and sqlite are OLTP DBs. It's going to be insanely faster for those use cases.

Actual databases pay up to two orders of magnitude of speed for durability. If you can regenerate your dataset in case the DB drops it (or there is a power outage, or whatever), being able to complete a complex, write-heavy query in 10s instead of 15 minutes is actually very useful when doing analysis.

If you want to use a database instead of Polars, for similar use cases, duckdb is a much better bet than postgressql or sqlite.

Because those are databases? Polars is a data processing engine, not a database. They have different uses.