For me, personally, I realized that having a system which I can take from zero to 'dev ready' in less than an afternoon (which I did recently with a freshly shipped MacBook) is the main 'feature' I want from my dev software stack. I don't love all of OS X's defaults, but I customize very few things.
The bigger issue for me, though, is hardware. I know this varies dramatically from developer to developer, but weight, size, battery life, and display quality (in roughly that order) are my primary concerns with laptop hardware. I recently switched my work laptop from a Chromebook Pixel (first gen) to an 11" MacBook Air and I've been thrilled with the change. That said, the leap in display quality and general machine 'feel' from the Air to the MacBook I use for personal developer is almost as big, for me, as the leap from the Pixel to the Air. Apple builds hardware that feels 'right' to me in a way that other manufacturers that I've encountered don't.
So, I can't speak for others, but there's why I'm a proud OS X user instead of a proud Linux user.
Another factor is that there a bunch of things people want/expect from a window manager that I never took advantage of, the biggest two being virtual desktops and focus-follows-mouse. I imagine if I had a strong dependency on either of those, the switch to a Mac would've been considerably more painful. On the other hand, features like Expose felt very natural to me (and at the time Linux equivalents were... poor).
These days my life consists almost entirely of running full-screen apps and trackpad swiping between them, which would probably drive most X11 users batty :)
Sound, oh sound. How many sound daemons did the community go through?
After getting Linux set up, I was always scared to change anything, because I might lose a whole day recovering my desktop. Back when spending a day messing with an OS was a learning experience, this wasn't a bad thing, but I learn absolutely nothing from it these days.
Also I care what my DE (Desktop environment) looks like, there are a couple of nice looking Linux DE's but they normally work for only around 70% of apps so you have a couple that stick out like sore thumbs. On OSX this only happens for things like some Java apps and even then they still look fine if a little out of place.
At my last work, my desktop was an ubuntu LTS version. It was rock solid, and ran as fast the day that office closed down as the day I set it up. The same could very much not be said of the macs in the office - almost all of which had slowed down considerably. One macbook with 4GB of RAM had zero free memory on a fresh boot with nothing loaded! I had to install some 'ram compactor' tool to get the laptop to actually load some office software in a reasonable time. Occasionally I'd get called over to help troubleshoot this or that with the various macs, and usually in the first couple of minutes, I'd be saying something along the lines of "how can you work with it this slow?". The mac users in this office were a mix of normal users and power users. The only expert user had a top-end model less than a year old, so he wasn't getting the same problems, of course.
So no, I don't agree that the base OS is ROCK SOLID compared to ANY linux desktop distro. You say you have better things to do than rebuild your entire desktop environment every few months? I have better things to do that wait for my desktop environment in day-to-day work. Unless, of course, I'm willing to throw gobs of money at the upgrade treadmill, which means replacing the hardware regularly, which means... rebuilding the desktop environment on the new machine...
It also integrates extremely well with other parts of the Apple ecosystem, unlike Linux.
Graphics is so much easier to deal with on OS X than Linux it makes me cry. I take my laptop between a 4K monitor on Thunderbolt, a 2560 monitor on HDMI, and a 1920 monitor on DVI. And I don't have to edit a single file on OS X. I shudder with the thought of that on Linux.
Networking just works on incarnations of OS X prior to 10.10. I can pop my laptop open and it pretty much connects anywhere with no configuration changes and only asks when I have to cough up a password. Linux still regularly makes me do wonky command line dances when the interface suddenly disappears or goes down.
I think it comes down to picking hardware that is supported well, i.e. Intel stuff. Apple of course for the most part knows it's hardware and doesn't have to include a gazillion drivers for graphics boards of all make and model.