If Apple sells OS X on the shelf, how many people will switch from Windows to OS X? Most people are used to Windows and the way it works, and don't wanna switch. Most businesses are entrenched in Windows.
I think this is the main reason for Apple not to sell OSX separately for non Apple PC's.
There isnt a big market for it, and Apple is doing great now, market share is rising, stock value is good etc. It would cost a lot to make OSX more compatible for PC components, and to have support for them, and if 50 000 people would switch to OSX, that isnt good enough to make changes.
Just like people already said: Apple is a hardware company. OSX software is excellent, but it's there just to sell the hardware (Mac).
To answer the title question - Apple is selling OSX for PC, just not for all of the PC components and manufacturers.
But I disagree on Apple won't ever do this part. Apple is known for doing drastic changes more then few times in history. If market situation turns worse for Apple, they will be forced to do some changes, maybe even open OSX to a wider set of PC components.
IMO it's highly unlikely, but it is possible.
Microsoft also doesnt officially support all of the PC hardware. They have a Certified for Windows program with WHQL.