Not scared; it's just not their business model.
Apple sells SYSTEMS, not software. The hardware and software are designed by Apple to work together. The result is a great experience for most users.
Apple would lose bigtime if they tried to sell OS-X for J-Random hardware-du-jour machines. Their business model is based on being a smaller player, for folks who want their particular feature mix.
OSX is just flavor of linux...
Business Model or not, OSX runs fine on the SAME intel hardware they charge 1.5X for.