How does Polars relate to DataFusion these days? There's no reason for them not to converge into a single ecosystem, is there?

It does not use datafusion. https://github.com/pola-rs/polars/issues/6197

Yes, but why?

I mean, it's very clear that the projects have a lot in common, even if building one on top of another directly is not a good idea.

In the issue, polars' creator says it is to reduce dependencies:

   I don't want polars to depend on on the datafusion crate, that is a complexity that is not needed.