Because too much money makes a company stupid. Look at Paul Graham's essay about what went wrong at Yahoo ( http://www.paulgraham.com/yahoo.html). Same thing happened at GM, too much money for too long turned the culture from creating the future to protecting sacred cows.

What's worse is these big stagnant companies often have access to very large pools of money from the public markets to buy up scrappy competitors and sap their innovativeness.