Because the user licence that comes with OS X prevents you from doing that.
Technically Apple could come after you for your 1 hackintosh box that you have for personal use but it choses not to (at least so far, but signs so far seem to suggest that they really don't care about homebrew).
What they *do* care *very much* about is people selling them on as ready made PCs with OS X installed, and you're right, there is no technical difference between the guy who does it to one machine for himself and the guy who does it to a hundred machines and sells them to his friends, but it's entirely up to Apple who it goes after for licence violations. It's not like a trademark issue where you must defend any breach or you risk losing the mark - they will probably have an economic vs brand protection chart where "personal use" becomes "hackintosh vendor" and you become a target.
Make no mistake though, you are always a target if you make a hackintosh - they just leave you alone because you're no threat to their business model. The NFL won't care if you tape the game, but it will care if you then sell that tape on ebay.
Ok, ok, bad analogy, the NFL will care very deeply if you tape the game and will froth and foam and rage about home taping taking the food out of the starving mouths of NFL players' children, but you see my point.