There may be some common programming language in them especially for programs utilizing specific hardware like the graphics card but they still have to be ported to each OS. Window, Mac and Linux are all basically running on the same hardware but that doesn't mean programs are all going to be available for each OS.Sunny Boy wrote: as Richard said, so they could then use many of the same programs.
Apple in the past has been mostly been used by professionals in particular graphic arts and video. Photoshop has always been available on the Mac and in the professional world Mac was/is the dominant machine. Same with video, Final Cut Pro is top tier and only available on the Mac. The problem with that is you can't make huge amounts of money serving just professionals.