Python from the start has always been willing to use other languages and tools if it made sense. It's a pretty common pattern (at least in the data science and research circles I've been in) to write the original in python to see if it works, and then to port it over to a lower level language (C, Rust more and more these days) and expose an interface to it in Python.

For things like dependency resolution, where you're doing a lot of heavy CPU operations, there is a lot of room for optimization in lower level languages.

because writing python is a lot easier for most people than writing rust but changing from pip to uv is like a one liner?

Changing your build tool to another tool (designed to be a drop-in replacement) is very different from changing your entire codebase...

existing code? swapping build tools is extremely different

Well because best tool for the job. There is no natural law that everything must be bootstrapped. Languages use LLVM for example.

There are obvious answers....

Why do you ask?