Stealing a phrase from the sportbike side of my interests.... "Because the street is not a track."

Yes, those who are all caught up in fixed-gear cult do tend to eschew brakes on their fixies.

However, I believe that a number folks like myself who happen to ride a fixed-gear (for whatever reason / purpose) and who have careers, families, and enjoy spending time doting on their grandkids will often times have at least a front brake on their fixed gear bike: perhaps a rear as well if it started out life as a road bike.

While it's true that you can jump/skid a fixed gear to an abrubt stop, doing so won't get you stopped as fast as or predictably as a front brake and it destroys your rear tire. Frankly, I'm not as concerned about trashing a $20 tire as I am of slamming into -- or being slammed into by -- something bigger than me.