Apple licensed OSX to clone makers in the past, but that was one of the first things Jobs put a stop to if I recall correctly. There's a strong thread in Apple's strategy to prevent computing from becoming a low margin commodity. This helps Apple with its image of high-end or well-crafted devices, increases profit etc. Which is a smart move if you don't own the entire market (but can differentiate yourself from your competitors)

Apple was profitable before the iOS devices hit and much of that was due to this kind of smart thinking. It reminds me of Nintendo, who, despite not dominating the market, still manages to be quite profitable (especially compared to their competitors) with a very similar strategy.

Letting anybody build any ol' system willy nilly would cannibalize Apple's profits, plus give them quite a bit more work since ensuring drive compatibility with random hardware would become a major priority.

As a user I think it sucks, but I have to respect that as a business strategy.