Which they don't - depending on what you count as being "Windows":
- The zip functionality in Windows Explorer is licensed from a 3rd party (DynaZip[0]) - Before it was removed in Windows Vista, Pinball was also from an external developer [1][2]
I'm sure there are countless other examples.
[0]: http://www.innermedia.com/ [1]: https://devblogs.microsoft.com/oldnewthing/20121218-00/?p=58... [2]: https://devblogs.microsoft.com/oldnewthing/20181221-00/?p=10...
Well a multi-billion dollar company with a-lot of resources can afford to do this from scratch. Unsurprisingly, those hobbyist OSes don't stand a chance and progress will be disappointing compared to Windows and macOS.
Most importantly, you forgot one more alternative operating system that does fit in the desktop ecosystem and that is Fuchsia. As we can already see the hundreds of Linux distros that are still floating around for desktop market share, they don't and won't come even close to macOS or Windows here.
Google is actually very good at 'using Linux' to step into the mobile market with Android and the desktop market with ChromeOS which is the head start that these distros should have done for a standard OS that general users can use, but failed to do so. Now it looks somewhat likely that Fuchsia maybe replacing both Android and ChromeOS by being one integrated and unified mobile and desktop OS for laptops and phones which they can control.
If there is going to be a new OS entering the desktop and mobile markets, it is almost certainly going to be Fuchsia.
Doing a modern desktop operating system from scratch is very hard and very resource intensive. But trying to reverse engineer one and implement it without having specs and defined APIs is way harder.
Microsoft needed tens of thousands of software engineers, UI specialists, software architects to implement Windows. Apple didn't have the resources to write OS X from scratch, they got many bits from FreeBSD, NetBSD and Linux.
So trying to replicate Windows functionality and have 100% compatibility with Windows won't happen unless Windows goes open source. Which I won't be surprised to see in the future if Microsoft owns the rights to all Windows code.