I think I get that part :) but realistically if anyone wants to run macOS software they are most likely macOS users already. Whereas containerisation is useful to aid with development and deployment. Have you come across anyone who runs production software on macOS? :)
One might develop a game that by some kind of a miracle releases for macOS too. So they way to run CI tests on macOS. Or they might target iOS. And use macOS build machines to produce builds. The world is not only about web, you know?
