Because Apple is ran by marketing fools. Their product is easier to use, uses less parts, is more stable, and generally is the envy of the industry. But, when a consumer has a choice of buying the lowest Mac with little/no software; or a top notch PC with scads of software; the user typically choses the PC.
I wouldn't say marketing fools considering the praise they get in the advertising industry. Its basically a price issue. People think they are getting a bargain when they buy a cheap PC.
Huh? Macs comes preloaded with a bunch of excellent software, including a superior operating system, Internet applications, word processing, graphics software, Quicken, iTunes, iPhoto, iMovie, QuickTime, utilities, etc. Is there some particular must-have application you know of that Mac users can't get?