It requires effort on Apple's part to make these things changeable and tweakable. That's effort on top of the effort to get it to "just work". They appear to have come to the conclusion that they get better results, however they judge that, by spending those resources on different work and features rather than adding reconfigurability to existing features.
Some companies do enough work to get the system working. Others get it working and then spend effort adding the kind of configurability you want. Apple does gets it working and spends extra effort on it's style of polish. I can't think of anyone that does both Apple's type of UI polish and high configurability. Each of these companies is doing what they feel is worth most to them and at least indirectly what it feels it can use to appeal to customers.
Each style offers the companies and customers trades offs between different values and everybody makes different decisions based on their preferences.