Because your leg is not designed to spin circles applying even force tangentially to your foots motion throughout. A brake is designed to apply a an even force to the wheel throughout it's rotation.

Even force = better braking.

Why would it be beneficial to not rely on brakes to control your speed?

Traction and control in the snow.

Even force may mean better braking but in the snow you don't want to be braking as much as you want to be controlling your speed and you can do this much better by maintaining steady, smooth pedaling on the fixed. Additionally, anyone using rim brakes finds them failing frequently in the snow and cold weather conditions (if you have stream crossings).

Trust me when I say I am not trying to imply riding a fixed MTB off road is the best tool for everyday riding. But when it gets dicey in the snow it is a better tool than the freewheel. When the trails get iced over, a fixed with studded tires is like having 4 wheel drive in a car.

I know snow is a rare thing in GA, but if you ever get the chance, give the experiment a whirl. I think if you did it you would see what I mean better than how I've tried to put it in words. I've read your posts and know you argue the side of facts and logic. Turn this one around in your head for a bit, you may change your mind.

even pedaling speed != even force.
As far as maintaining traction goes it's better to apply a steady gentle force via brakes then an uneven one via pedals in my experience. I have not done it fixed but I have plenty of experience mtbing in the snow... I have not lived most of my life in ga.

Maybe I will try it some day but I people claim that it's better in other poor traction situations too and with the exception of ****ty brake failure that doesn't seem to be the case either.

I'm putting my money on the guy who's ridden a bunch of mountain miles fixed rather than the guy who has yet to even try it.

Thats you, teammate. Mondo Guano!

If I was doing it just for the hell of it I'd be the first to admit it. Because come summer time, if I ride fixed off road it's not because it's the best tool.

Come on up to MD, we've got a good testing ground right now.

Its in the plans and overdue.