From my perspective, Windows is an inferior system to any UNIX-based system on almost every front. It has a very strong application ecosystem, but technically it's unpleasant to work on (for me). So, given that, if there's something I want an OS to do that Linux can't do, I would be motivated to work on making Linux able to do that thing so I could do that thing on an Open Source OS I enjoy using, rather than work on replicating Windows so I could do that thing on an open source OS I don't enjoy using.
But, it seems there are people who genuinely enjoy Windows more than Linux (or BSD), and are willing to work for 17 years in pursuit of being able to run an Open Source Windows version. I admire them for it, but still can't understand it.