Because if you are going directly at Megacorps lunch, they kill you. Google did not build an os or office suite, they built a product in an area where MS was not looking, which is the same play MS ran on IBM. I'm not into business gurus that much but Clayton Christensen codified a lot of how those patterns play out; one of his conclusions was that going directly at an established cash cow of a large company is not a good idea unless you intend to sell.