> It’s the obvious way to implement backend resolution.

If a compatible version is already present, yes, a frontend can just use it if it's within the semver range specified.

But in many cases, builds are done in a fresh virtual environment that has nothing installed. In such a case, the only option is to download the latest version that's in the semver range specified. That's how the "build" frontend works, for example, unless you specify a flag to keep it from creating a fresh virtual environment.

Is that what Gentoo is doing here? I thought the whole point of the distro doing any sort of Python packaging is that they can pierce abstractions like virtual environments where it suits them.

(But also: if we’re talking virtual environments then we’re firmly in Python land, not distro land, where the correct answer is “use a wheel” instead of building it yourself.)