Because Xcode. Some of us need to iOS apps too even if it's not our first priority. At this moment the only sane way to do that is having OSX installed on a Mac.
Developers for all kind of stuff use OS X. Any conference, from Google IO to Oracle or Apache (Java) stuff, to RoR have tons of developers, the majority actually, running OS X. Even Rob Pike uses OS X. Even Linus Torvalds (although he just uses the Mac, not the OS, of course).
And most of them do not deploy anything to iOS.
The use OS X because it's a fine UNIX and it has all the commercial stuff they want to use (Adobe, Office, etc) and all the multimedia stuff they want to use too. And it has nice hardware to boot (not CPU or price wise: overall quality wise, from sturdiness to weight, and from display quality to driver support).
