If you use a directive antenna to concentrate the radiated power into a small solid angle, to reach a distant receiver, you also increase in the same proportion the interference for another receiver that is located in the same direction as yours, but which does not want to receive your signal.

So limiting EIRP provides a limit for the interference suffered by a receiver that happens to be in the direction towards which you transmit, for which it does not matter at all which is the total power that you transmit in all directions.