but there's probably lots of projects that use uv, but not ty.
I'd imagine that, for a substantial percentage of these, part of it is to avoid negotiating and bargaining over an additional tool in your CI arsenal, especially in bigger corporate environments where these sort of conversations are dragged through layers of non-technical middle management.
Ty isn't a compelling option. If you want support for dynamic libraries, mypy is king. If you want speed and strict conformance, Pyrefly and Zuban are better.
Ty's original pitch was that it would have strong inference and be useful for gradually typing existing, untyped codebases, but AI made that moot. Adding types to existing code is now trivial.