Projects that use ty for its type checking but something else than uv for managing dependencies are probably in a very tiny minority of users.
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.
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