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.

They would sell because their product has no demonstrable value at this point.

Wouldn't that be the worst time to sell? Wouldn't you be able to negotiate a higher price with a product with demonstrable value?