The reason you don't is that in order for it to work, twice as many frames need to be generated, one for your left eye and one for your right, in order for a game to appear to run at a certain framerate. So if you want a game to look like it runs at 30fps, you need to actually render 60, 30 for the left and 30 for the right. This wouldn't be an issue if you could simply double the number of frames a console can put out, but that's not how it works. Functionally what happens is that the effective framerate (what you see) gets cut in half. So FC3 in 3D on an xbox would run at 12 fps with dips down to 6 or 7 fps. Halo 4 would run at 15 fps. Call of duty can manage it since those games run at 60 fps.
tl;dr the xbox is old and weak in the knees and the hardware isn't up to the task.
So if next gen is as powerful as we come to expect it to be then handling 3D for every single game wouldn't be a problem right?
No. It's never a question of "how powerful" a system is, it's all about whether developers are willing to make the concessions required for 3D to be enabled on a game. True stereoscopic rendering is nearly as demanding as actual splitscreen is, so if
That said, something like Trioviz 3d is extremely low-cost to implement (both in actual effort required and in performance hit), so I wouldn't be surprised to see that become commonplace if it's an often demanded feature.
Oh, and yes, as previously mentioned:
And just for the fun of it, Tomb Raider 2 in 3D back in 1999:
"Walking tanks must exist somewhere for there to be such attention to detail like this in mech sim." - IGN Steel Battalion review