They could have a million employees, if they are all tasked on making one game in a 3 year cycle, then thats what they will make.
And Bungie's one game has sold more than Epic's 3 (there's really only 2 yet tho isn't there) so far, so maybe that's the right decision for them.
Blizzard with more employees than all of the above put together manages about 1 game every 3 years, with maybe an expansion thrown in now and again.
I'm not saying that Halo 3 is a bad game, just that due to Bungie's choice to use the Halo 2 engine, instead of starting from scratch like Epic, Infinity Ward, Insomniac etc it lacks the visual prowess its Xbox predecessors did.