Probably because they don't want to self-host Windows or MacOS servers when they can pay someone else to do that for them (or Linux ones, I assume that is within their wheelhouse for production but CI is a bit of a different beast to model inference).
Presumably because they "build for [macOS, Linux, FreeBSD, Android, Windows] x [ARM64, x64]" and self hosting all of that would be time-consuming and expensive.
Probably because they don't want to self-host Windows or MacOS servers when they can pay someone else to do that for them (or Linux ones, I assume that is within their wheelhouse for production but CI is a bit of a different beast to model inference).
Presumably because they "build for [macOS, Linux, FreeBSD, Android, Windows] x [ARM64, x64]" and self hosting all of that would be time-consuming and expensive.