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.
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.