The long stems are a function of riding smaller frame sizes. The major reason to ride a smaller frame size is that it results in a lower position, i.e. its easier to get a significant drop from the seat to the bars with a 56cm top tube, and 120mm stem, than it is with a 58 top tube and a 100mm stem.

Other reasons, stiffness and weight don't really matter for tDF riders ( you can make any frame acceptably stiff theses days, and they're all at the UCI weight limit anyway)

So the answer is aerodynamics.

As for "S turns" with pedal strokes, that happens to everyone riding a bike. Take a look at yourself with an aerial shot. You just don't notice it, without that viewpoint.