It's hard to explain without graphics or a diagram. You are faster aerodynamically with a flattened back (actually a back with a slight hump like a teardrop). The way to get that is being forward on the bike with your arms outstretched. With a traditional road bike geometry, you are limited in how far you can be forward, in part, by the seta tube and headtube angles.
Part of that can be overcome on a road bike by using a forward leaning seatpost. Scooting the seta forward as you said also works. But the problem with both is you find your knee too far ahead of your ideal foot position on the pedal. That sets you up for potentail injury plus doesn't allow for optimum power over a variety of situations. You can also use a longer stem but the handling starts getting bad. You can also scoot your seat forward