I imagine at least some part of it is that acquisitions are difficult, time consuming and rarely work 100%. There are almost always seams after the fact and problems with culture integration. Cash on hand also gives you the option to respond to market swings and changes. It allows you to invest in new opportunities, and to build a space organically from your own internal visions. Would the iPhone been what it was if Apple had just bought Nokia, or Ericson or any other phone vendor at the time? Maybe, but I strongly suspect not. And at least for Apple especially, the point isn't just to have the product, it's to have a well and tightly integrated product, and existing product lines might need to be so heavily re-worked any value in buying it as is would be lost in having to scrap so much of what's where.