Did you actually try it? Across Windows, Linux and macOS? Setting up a vanilla Clang toolchain for cross-compilation is by far not as trivial as you make it sound.
You mean like cross compiling from Windows to Symbian, Windows CE/Pocket PC, Palm, Android, PlayStation, Nintendo, XBox?
It is not a walk in the park, but it hardly a task only worthy of Hercules.
Nobody said it was a task only worthy of Hercules. Just that it is a royal pain, and Zig removes that pain. You asked "why zig?". That's your answer.
And then did you try the same on Linux and macOS? That's what the Zig toolchain gives you, it doesn't lock you into a specific OS.
Yeah, and zig solves that. Instead of installing, trying the right header files, you just untar 70mb and you are good to go.