Originally Posted by
flaco
Saddle position is limited by UCI rules, so when they're forward on the aero bars, they often don't have much saddle left.
Ding Ding Ding, this is the main answer
There is a hard compromise between aerodynamics and producing power. Triathletes fix this by moving there saddle forward, as much as to have the nose "in front of" the bottom bracket, which maintains an open hip angle. UCI constrained cyclists have to sit as far forward on their saddles as possible to achieve the same effect.
It's not terribly uncomfortable to do this when you are going hard and racing under one hour. As some pointed out, it's also an effect of the effort and getting weight on to the legs.