There's only so much developer time and management focus. "This will help us cement our monopoly on software distribution without being too obvious to regulators" is a good pitch.
PS. I'm sure the people who pitched it and their managers are all sufficiently skilled at lying and lying to themselves to not put it or even think in those terms.
You can ship macOS native games on Steam as well. People need to stop throwing conspiracy theories. Their main motivation is to make sure games are native to macOS so they can take advantage of system-native features, which Win32-translated games won't. Otherwise games running on Macs will always be kind of janky and run slower than Windows.
