To answer your first para Tbg - familiarity and control are the main reasons - but also the wealth of free applications for the PC. Now I am learning more about Unix I can see that you can get a lot of control with Macs too. But I can't help myself, I prefer to build my own Mac than buy a Mac - I, of course, use the retail software. In particular, having zero moving parts (the music is stored externally) and a fair amount of power and 8GB of RAM is beneficial to sound and no Mac quite meets that spec.
On the software availability side, for example, I use a DSP board I have heavily modified out of a Rane RPM 26z in a system where I am designing an open baffle speaker. The Rane can do a lot of DSP stuff, but I really just use it for the digital cross-overs. The Rane software is just brilliant for real time design changes from the listening chair via wifi and a mouse. It is great for hearing immediately the change from 12dB/octave slopes to 24dB, for example. But it only runs on a PC.
Similarly I use TrueRTA for in-room measurements - free on a PC. Were I to use onboard crossover software on the Mac and output through a Lynx card, again, there is readily available software for the PC. Maybe I am looking in the wrong places for Mac software, maybe its there and it is just expensive, or maybe I should look at Parallels.
But the more I use Unix, the more I like it, I have to admit. I was put off initially by Macs because Apple wants to make all your decisions for you, but I am learning how to get under the hood now.