I have 120mm stems on both of my bikes, so that makes me cooler than everyone, right?

yes

Using risers on road/track frames amounts to nothing more than a hack. The engineers that desinged these frames (or the frames that current frames are based on) did not have risers in mind.

The only bikes that are designed to be used with risers are MOUNTAIN BIKES...and maybe a few city bikes.




Modern fixed gear bikes are pretty much franken-bikes with:
- Track frames
- Track wheels
- BMX or antiquated track pedals
- Antiquated toe straps or velcro straps that serve no other cycling market (except a handful of track sprinters).
- MTB riser bars
- BMX/MTB stems
- All to be ridden as commuter bikes

So that's why.

+1

Time Trial bikes use shorter stems because riders are on their forearms in aerobars. That's the only exception.

EXACTLY.

I'd be interested in what the biomechanics are though. Is it just to get more weight over the wheel or is there some reason that a longer stem provides more torque or what? It seems like if it was getting weight over the wheel then we would expect road bikes to have longer stems overall than track bikes, because of the rake.

It's a matter of handling at high speeds.

Without getting into too much detail (some of which I don't know the science of), generally speaking, the objective is to have your hands above the front axle (actually slightly behind...like 1cm) and subsequently your hands will be over where the wheel touches the ground.

At higher speeds, Bikes handle really twitchy the further the hands are behind the front axle. And it handles sluggishly when the hands are in front of the axle.

Ever get on a townie bike with bars that are swept back? It's really weird steering those because of that reason. Take that bike up to 25+ mph, and its REALLY tough to steer.

Notice the hands slightly behind the axle: