Im my experience aside from racers most riders generally ride on the hoods or the top portion of the bar so that is where the shifters are designed to work from. Actually I don't think most riders realize they can actually take their hands off the hoods once they get moving, they seem to think it is possible to move them even for quick wave at another cyclist. but I am off topic.

I think it is just a pratice and technigue thing. I seldom ride in the drops so I never really thought about using brifters from there. "Back in the day" as they say some serious 'Crit' riders used a barend shifter for the rear since a crit is generally the only time your on the drops significantly.