Profit margin sure is a big difference, and also Apple makes more profit in one quarter than Amazon has EVER made in its entire company's history. Amazon is in the retail business, and it's razor thin profit margin when compare to traditional tech. Apple simply CANNOT funnel that much cash back into the company, unless you consider paying absurd amount of money buying over-priced social networks a smart investment.

Amazon and Apple are very different companies. Apple designs, manufactures (well, mostly contracts out manufacturing), and sells a relatively small number of relatively high margin products that they sell boatloads of. (And, like Amazon, they have some cloud services but at Apple's scale these are a pretty small piece.)

Amazon on the other hand continues to build out enormous infrastructure both for Amazon Web Services and Amazon.com. AWS is also in something of a price was with Google right now with various headlines today somewhat hyperbolically predicting that cloud services were headed towards essentially free. That said, Amazon is arguably building a huge (and costly) moat around its business.

They're both impressive companies. But Apple's business requires a whole lot less reinvestment relative to revenue both now or in any near- to mid-term foreseeable future.

Nature of the business. Amazon is a growing company and invests heavily in itself to stay relevant, to grow, to achieve long-term success in a very competitive environment. Amazon's customers care mainly about price. Apple has been successful in high-quality innovation and has created a remarkably valuable brand. Apple's customers care mainly about quality and are willing to pay a high price for it. Also, geographies: Apple has achieved far greater economies of scale than Amazon, and Apple will continue to enjoy the benefits of global scale combined with high margins so long as its products stay ahead of the competition.