While I agree that they use many of the same suppliers, you're not going to find the same graphics cards (for example) in an Apple computer that you would in a generic PC. Where the PC cards tend to be optimized for Direct X, Apple uses OpenGL which, while they can run on the same cards, don't need all the same processor calls. As for hard drives, RAM, etc you'll find that again Apple tends to require certain accommodations. Yes, you can get away with generic parts in many cases but in the case of RAM, etc. you'll find that Apple's choices simply last longer in full-time use.

I'll go deeper, having worked for an Apple supplier in the past. Apple will order supposedly commodity parts from a given supplier, yes, but their contract will demand extremely tight specifications on those parts--to the point that the manufacturer has to perform 100% QA testing before they ship and even then if Apple's own random testing reveals a failure rate beyond a certain percentage the entire load will be returned for re-testing. Why do you think Apple switches back and forth between video card and hard drive suppliers so much? If they have to repair/replace a given component too many times they call it a breach of contract and go to a competitor. Since there are only limited competitors this means when one company fails, they go to the other. Apple is in the process even now of weaning itself from an unhealthy reliance on Samsung despite the fact that Samsung has provided the best quality parts for years--to the point that Samsung is using its knowledge of Apple's orders to build competing products and releasing them within +/-2-3 months of Apple's releases.