Earlier quoted context omitted.
If it was just about apps, why wouldn't they release OS X for all x86* architectures instead of just their hardware? Then projects such as hackintosh/osx86 would be irrelevant.
Increased support burden, by several orders of magnitude. People blaming the OS for hardware deficiencies. Etc. I'm not seeing any upside for Apple if they did this.
This is also the reason they released Mav for Macs going back to 2008. To reduce the support burden of previous OSs