The reason most packed up and left by 9 is b/c ducks mainly fly in the wee hours of the morning. This is not to say that you won' t see any after 9, but you will more than likely see less as the morning progresses.

I do exactly the same thing when I duck hunt. I am usually out of the area by 9 or 10 at the latest. The first hour of light is by far the best where I' m from. Just my observations from VA.