Probably the same reason there are treads on most road tires - its what consumers are used to seeing. TT bars are proof positive that you don't need drop bars to go aero. It realy depends how you set up the stack height, stem, and the kind of flat bars you're running. I have horizontally oriented Ergon GC3's on a hybrid tourer and the bar setup is slightly lower than the tops of the drop bars on the Specialized Roubaix and gives about the same riding positions as the hoods and top bar on that bike.