Because the working theory on what causes simulator sickness is related to the inner-ear detecting motion. If your eyes are detecting motion, but it does not line up with what your inner-ear pressure is telling your body, the end result is the body emptying its contents under the theory that it's been poisoned. Simply improving the quality of the picture isn't going to solve that.
It seems to me the oculus rift can accrately account for all head motion when you are standing still. There is no need for eye-tracking: All the rift has to do is detect all head bobbing & rotation, and update the view acocrdingly. Your eyes can still look around on the lcd screen.
What the rift _can't_ do is account for long term linear acceleration (eg in a car or walking around), it's true. If you had a large empty room to walk around in with the rift on, even this could be accounted for, however.
