I don't know about Apple but Facebook and Google have pretty big offices in Seattle as well, along with many other startups (Uber, Airbnb, Stripe, etc.)
It seems the play these days is to have the best of both worlds.
Companies at that level generally have offices in big cities all over the world. In Toronto we have Airbnb, Uber, Google, etc offices for example. I think the important thing in this context is where the HQ is.
And Microsoft and Amazon both have big offices in Silicon Valley. Bing is (was? it looks like the site is now under reconstruction) based out of their Mountain View complex that's almost literally across the street from the Googleplex, while A9 (Amazon's search subsidiary) has a big office in Palo Alto.
It seems the play these days is to have the best of both worlds.