I think they start that early for a couple reasons. First, weekends are usually the only day in the week when full time workers can get in long rides. If they start aroung 10 like you suggest, they wouldn't be back till late afternoon, and most people have familly responsibilities that don't go away just because they have to ride. Also if the people race, they need to get their bodies used to performing early in the day since most road races start at the crack of dawn, not taking into account the drive to get to the race which puts people in their cars at O'dark early.
The other reason for the early starts I think is to get people to show up. If you put it later int the day, stuff pops up that hampers someone from getting out the door. If your ride is the first thing on your agenda in the morning, those kind of things won't pop up and detract a larger group from showing up. The early starts also avoid the heat of the day once the spring and summer roll around. If people are used to the ride starting at 8:00 all year, they won't forget the start time due to season changes.
For the above poster who commented about the coffee, try diet coke. It would curb your bodies desire for caffine, and still not sour your stomach. If you are getting headaches from a lack of caffine in the morning, you might want to look at weening yourself from the joe a little. Caffine is a powerful stimulant that can become a dependency. Not that you will end up on the street sniffing junk behind a dumpster from a coffee habit, but it will effect your training.
