uv does not ship system libraries because pypi does not have them. There is a philosophical difference between pypi and conda today. I believe over time pypi will likely ship some system libraries but we will see.
It doesn’t “restrict” to pypi but it wants to be rooted in pypi. That means if you install “tensorflow” you get it from there. The state of pypi is the state of pypi and I have some hope that this also will improve. See for instance the efforts that go into “wheel next”.