Well, mainly because it's a better unix than Linux for the desktop, and I'd rather pull my eyes out of their sockets with a rusty screwdriver than use Windows.
Other than developing my own (without using any other OS...) which is a ... significant ... task, there's not much other option. YMMV.
It might be "better unix" (whatever that means), but it sure as hell is not better. Locked down, buggy, and difficult / impossible to navigate by keyboard. And I need to install (and trust) a 3rd party app to get a multi value clipboard? Yeah right, better. I'd prefer Windows, and I'm not fan of the ad-OS either.
MacOS is a better desktop in the sense that the desktop is locked down. GNOME trie to be the same as MacOS but being the default desktop for nerds and build for people who lives the Apple way makes it a bit schizofrenic.
As a Linux lifer I agree that the hard diamond surface of the Mac desktop has a solid feeling to it. The Linux way is harder and also more brittle. Windows and Linux are both better than MacOS even as a desktop as long as you do not look at the in the wrong way. The thing is I have only minor problems doing that on either Linux or Windows, but the walled garden of the Mac, Android and iOS is a joke.
MacOS is designed to be a somewhat stable desktop, that is good. It is not a better Unix, it is a political stance that means hacking will forever die.
Better is subjective.
It was a better linux for the desktop back during the snow leopard day but it's slowly gotten worse at the same time that linux became better. Now the only advantages they have is the hardware. The os is buggy doesn't respect apple's own human interface guidelines and is increasingly locked down. Gone are the days of simbl extensions, customizability and a clean nice coherent stable os with few bugs.