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.
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.
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.
I don’t know anything about “hard/brittle” analogies for operating systems. What I do know is that Linux distributions don’t seem to have a coherent strategy for building an operating system with sensible defaults and a consistent design that makes it easy to use for non-technical users.
Linux developers seem to almost-universally believe that if the user doesn’t like it or it doesn’t make sense then the user will fix it themselves either via configuration files or patching the source code. That model works fine for users with a lot of knowledge and time on their hands. In other words, it’s an operating system for hobbyists.
MacOS, for all its faults, is still pretty easy to use (though not even close to the ease of use of Classic Mac OS 9 and earlier).
Apple developers seem to almost-universally believe that if the user doesn’t like it or it doesn’t make sense then the user will... just have to learn to live with it.
I never said the Mac was perfect. Far from it. But it has sensible defaults which the vast majority of users find acceptable and easy enough to use.
Linux users, on the other hand, seem to spend more time customizing their operating system and sharing screenshots of it than actually getting work done.
You are encouraged to play with footguns on Linux, I do not do it and none of my family do it works fine for us. On "Linux desktop" one of the things you are not encouraged to play with is installation of programs. The Linux way is preferable that is why Apple and all the other are walking down the same path.
Not being able to install things sucks, but when you do you will easily destroy your nice shiny brittle desktop. The pebkac is strong here, but making the users enemies is a bad solution, this is why Google, Apple and MS are all bad desktops.
As I said I have been a Linux user my whole life. I know it works as a desktop but it works best with either people who do not care about instaling stuff, or thise who care enough to get it working.
Better is subjective.