Because it seems as if people ignore what PC means - personal computer.
So - is a Mac not a personal computer? Does it not have the same bits and pieces (keyboard, mouse, CPU, screen, disk drive) , and do the same functions (email websurfing, program running, photos, video, etc )? It even has the same processor manufacturer (Intel)!
So why is a computer that runs Windows a PC and a computer that runs OSX not a PC? what about Linux OS computers that do the same functions? Are they yet another term?