why doesn't Apple open up OSX?
Because they do not want to. Just like Microsoft doesn't open up their OS. They don't want to nor are they under any obligation to do so to meet the whims of the market. There are operating systems out there that advertise themselves as open. They readily compete in the market and Apple and Microsoft do not stop people from using those systems.
There are only two ways that Apple distributes their OS:
1) Preinstalled on a Mac (Apple labeled computer)
2) As an update with the requisite being that you have part one and you meet the system requirements.
Apple does not sell a full version for generic computers nor do they sell an OEM version just like they don't sell a jewel encrusted Mac despite people wanting it. Apple does not have to accommodate markets that they do not wish to sell it to. Just like you cannot buy the ProTools software without the hardware to accompany it.