IT's not a matter of "keeping the good stuff" - it is about OEM specifications. Even with Apple's relatively small market share - you can bet that if Steve Jobs calls up ATI (or nVidia) and says he wants x-model video card - but wants this connector, this gpu, this memory config, and this particular board configuration, they won't accept and deliver? We are still talking about millions of $$$.
My current desktop machine - an Apple MDD PowerMac dual 1Ghz- came with a Pioneer "SuperDrive" that has worked flawlessly. In comparing it physically to the other OEM and retail packaged versions of the same drive, there are minor differences. Apple specifies some firmware that may restrict certain features of the drive that may be problematic.
Also - Apple's implementation of said hardware (stock/OEM) also enables tigher integration - which causes less conflicts (I have heard that up to 90% of warranty/service work on PC's is actually software related or caused). Reduce the software-related problems and you have made a significan dent in the service calls.
But even when adding OEM and Retail packaged add-ons (like additional hard dries/PCI cards/etc.), I have not encountered the same trouble that some PC users find.
Maybe the component suppliers are not often the trouble - it could be that the specifications by Apple are tight enough on the assembly end, that things just work better --- who knows.
What really is important - owner satisfaction - and Apple has long been a leader.