Also it's because shorter reach with more elbow bend is better for aero than longer reach with straighter arms. In fact the most aero position on a road bike is not in the drops; it's on the hoods with elbows bent such that forearms and torso are parallel to the ground (or nearly so). This more modern fit style also allows riding a slightly smaller frame, which will be a little lighter/stiffer. If you look back a few decades, roadies were riding more stretched out and spent more time in the drops, this is a fairly recent development.