I can't tell you why they SHOULD, but I can tell you why some might want to:
Personal OS/Interface preference. I've used Windows 2k and XP for years. I find the OS X interface to be cleaner, easier, and faster to use. I just prefer it. Just like I prefer my new car over my old one. No logic, no stats, just what I feel happier/more comfortable using.
OS Level ColorSync support, which extends through screen calibration, printers, Safari, Preview, Photoshop, etc...
Built-in screen calibration. No, it's not as good as a $ hardware/software package, but it's pretty good, and good enough for web work certainly.
The ability to run any OS/browser combination easily (Parallels or VM Ware). Macs aren't a majority, but they're up there, and depending on your site's demographic, making sure stuff looks right/works on the Mac can be very important. I run OS X/Safari, OS X/Firefox, WinXP/IE6, WinXP/IE7, WinXP/Firefox, Vista/IE7, Vista/Firefox, etc... on my Mac very easily. I can also run my Windows VM at whatever screen size/resolution I want to see how things will look on a 1024/768 display, without hosing my main display/work area.
For whatever reason, using Photoshop on a Mac is easier/better looking for me. I can't back that up, but it's a strong difference for me.
Apple displays generally have better color gamut, than the displays from major PC vendors. Even the photographers I know who choose to use Windows (which is fine), usually use Apple displays, unless their using a very high buck non-consumer display.
I'm sure other folks are much happier using PCs, which is fine. These are just some of my reasons.