Why must the citizens of Oceania never be allowed to set eyes on citizens from Eastasia or Eurasia?

"If he were allowed contact with foreigners he would discover that they are creatures similar to himself and that most of what he has been told about them is lies. The sealed world in which he lives would be broken, and the fear, hatred, and self-righteousness on which his morale depends might evaporate. It is therefore realized on all sides that however often Persia, or Egypt, or Java, or Ceylon may change hands, the main frontiers must never be crossed by anything except bombs."