Gentoo builds every package from source on every user's machine, and when each user does an update, every package that changed gets rebuilt from source again.
So every Gentoo user that has a Python project that uses uv-build ends up having to rebuild uv-build, and then every Python project they have that uses it, every time uv-build pushes an update.
Other distros don't necessarily have quite the same problem because, if they include packages of Python projects, they don't normally track the build backend as a dependency. So if uv-build pushes an update, the distro would push an updated package for uv-build, but they wouldn't look at every Python project they package to see if it used uv-build as a build backend and rebuild it if it did. It's the "rebuild from source every time" aspect of Gentoo that forces that connection.
So every Gentoo user that has a Python project that uses uv-build ends up having to rebuild uv-build, and then every Python project they have that uses it, every time uv-build pushes an update.
Other distros don't necessarily have quite the same problem because, if they include packages of Python projects, they don't normally track the build backend as a dependency. So if uv-build pushes an update, the distro would push an updated package for uv-build, but they wouldn't look at every Python project they package to see if it used uv-build as a build backend and rebuild it if it did. It's the "rebuild from source every time" aspect of Gentoo that forces that connection.