Errr ... because they're commercial operating systems, with the expectation that software is not just universally available for no cost?
The respective stores for macOS and Windows are changing this, but there's a 30-ish year history of needing to register and pay for your software, so having it all just _there_ still isn't really a thing.
The respective stores for macOS and Windows are changing this, but there's a 30-ish year history of needing to register and pay for your software, so having it all just _there_ still isn't really a thing.