Perhaps the biggest reason is that the existing FOSS OS world has little to offer for people who like a Mac-style desktop. It's possible to customize DEs to have a very shallow visual resemblance but the details are all wrong, with Windows, legacy Linux desktops, and even mobile operating systems (in e.g. the case of GNOME) exerting the greater bulk of design influence.

The question might then, "why not make this a DE instead?" to which the answer is that Mac users also like how the underpinnings of the OS work (or have worked in older releases) and would like to continue to use their favorite Mac apps, which isn't going to happen as long as any traditional Linux distribution is involved. Mac apps also have a design ethos that's rare among current native Linux apps, which drives demand for alternatives.

To do this properly it really needs to be a whole-OS endeavor.

> when you could run Darwin

That's what this is. Darwin isn't installable today. Apple just dumps a bunch of source code. Previous attempts at making Darwin bootable didn't go far and had no GUI.

I dunno you could definitely do a "wine but for macos" kind of implementation, e.g., https://www.darlinghq.org/ without breaking IP laws

I've always been shocked by the lengths of time people will commit to proving wrong the folks who tell them "you can't do that."

why do anything? why Linux? why FreeBSD? why Mac at all? Why build your own wind turbine when corporations make large ones? Why build a cyclekart when you own a car? Why workout when everyone's going to die?

With that mindset, everything is useless and nothing has a point. Have I made my point?

... or flowers for the grave? :)