Because you can do things on a mac that you cannot do on 'easily' other hardware...
Let's say for example you are running Xcode, macs have the ability to run as a 'farm' service, that is to say that if you have a corporate network with macs, each and every one is able to share the processing power of the others and it is built right into the applications.
Just plug the shit together, its all automatic.
You are wrong. There is no "automatic farm service" in Mac OS X.
If you are referring to the ability in Xcode to do distributed builds, well, it is a feature built specifically in Xcode, not in the OS. If you are referring to Grand Central Dispatch and the C extensions to do automated threading, well it is constrained to a single machine, there is certainly no "farm" support.