Cool question. My theory: I think it's because they have phenomenal market penetration- so many people use Google search, Gmail, Google Maps. The term Google (to me, at least) elicits a microcosm of the Valley at large. Do they have the most employees, or the most political will? I'm thinking of "Washington" as the shorthand for "USA", or "Wall St." as shorthand for "New York"... maybe I'm wrong, but it's an interesting thought experiment to sit and evaluate all these "X is the default shorthand for Y" statements that get thrown around.