Why have people a right and left foot?

Symmetry basically, the fact that the toes nearest our center line are larger and stronger is because we would have used those to grip with when we were monkeys living in trees before we evolved to move around solely on the ground. The reason why the stronger toes are centered is that the most efficient way for weight to be supported is if it is suspended along the centerline of the object that is hanging. When we began to walk our weight was still heaviest in a centerline down our mass and therefore it still made evolutionary sense to have the strongest supporting toes in centerline with that mass.
If we had two left or two right feet the weight distribution of our bodies would be uneven (either if we hung from branches by our toes or walked on the ground) and could not have developed as it would have been evolutionarily unsound, that is it is not a trait that confers as much of an advantage as the arrangement we actually have.