Because I don't want Linux. MacOS largely still just works and Linux is moving away from its strengths as quickly as possible. I don't want systemd. I don't want pulse audio. I don't want Wayland. Linux's strength, for me, has been that it's unixy. If I'm moving to a non-unixy setup, like Linux is becoming, I want it to be as polished and well supported as possible. Linux, simply put, is none of those.
And, quite frankly, I don't want a Thinkpad (and I can't stand that eraser tip pointing thing). I bought a 2015 rMBP late last year and cross shopped offerings from Dell, Lenovo, and one of those Linux laptop companies whose name is escaping me right now. The Mac won out and didn't command a premium over comparably specced alternatives.
I'll spare the ranting about the Gnome 3, new GIMP UI and systemd in general (but these are both major issues for me and symptomatic of a culture I don't want to buy into). I recently tried to set up a Pi Zero with an Audioquest DragonFly DAC. The DAC just works in FreeBSD and MacOS with no hassle. I was able to tweak ALSA to get sound out of it, but shairport-sync can't set the volume (which works just fine on FreeBSD). WiFi was a struggle as well. I forgot what an archaic mess of procfs tunables and bizarre config files Linux on the desktop can be, and I don't want to spend my days debugging my desktop system.
One of my coworkers at megacorp got the officially sanctioned Linux laptop (some Dell thing). Like a good end user he installed updates as they became available. Turns out megacorp bought the Windows version of the laptop and installed Linux on it. Well, my coworker installed some firmware updates that promptly re-enabled secure boot borking his system. He lost a day trying to figure out what happened (Dell wouldn't support Linux on it and corporate IT was entirely indifferent). I don't want to dodge a minefield masquerading as a support matrix.
It seems like it's been the year of the Linux on the desktop for most of my adult life.
Edit: Downvotes don't win over users.