The actual reason is... on concrete runways there is usually a pavement joint running right down the runway centerline.
For structural reasons runway lights must not be installed at pavement joints, so the lights must be offset a minimum of 2 ft and a maximum of 2.5 ft from the centerline joint (FAA requirement). The direction of the offset is up to the designer; it is usually selected to minimize equipment / wiring costs.
For official specs see
FAA AC 150/5340-30E, Chapter 3. "RUNWAY CENTERLINE AND TOUCHDOWN LIGHTING SYSTEMS".