As other comments have pointed out, a comparably specced PC is not that much cheaper and would be much more of a hassle.
During those six and a half years I knew I could count on this machine and I never had any annoyances with it. If I had gotten a PC which was unlikely to last as long, I would have had to change it at least once, which means going through a period of not quite broken but not quite hassle-free either. And two PCs with the specs of my MBP would have cost more. I'm also OK with paying a premium to not have to deal with those issues (noise, screen sometimes not working, ports failing, keyboards dying, etc — all issues I've had with "pro-level" computers, mostly HP).
There's also the fact that at the time a similar PC was extremely rare (maybe the X1?). I'm talking about a computer with thunderbolt, high resolution display, the ability to run a 4K display at 60Hz, fast SSD. Granted, all those are much more common today.
I'll refer you to the other thread trending today about a blogger who upgraded from the Macbook Air I have to a new Macbook Pro and doesn't like it.
I also have Dell and HP equipment, and most of them have weird driver issues with the basic hardware after about 2 years. I don't want to live like that... I currently have an i7 Dell laptop that randomly disables the keyboard based on which WiFi network it's connected to. The other Dell I have spent its first two years of life re-installing the wrong SSD driver with every Windows update (when it crashed, I'd have to boot into safe mode and re-install the correct SSD driver that I kept on a thumb drive).
So, yeah: Dell is out. Looking at other brands, and desktops.
A lot of manufacturers offer aluminum bodied laptops, though I'm kinda biased towards the feel of HP laptops
My current 2013 model does have its share of scratches and two small dents but it works as well as 6 years ago. I'd much rather have a dented metal case that stays together than a cracked plastic one.