The "too" is the point here. Yes in the past their would be less avenues for them to get back in contact if you went away, but that going away would be much mo

The "too" is the point here. Yes in the past their would be less avenues for them to get back in contact if you went away, but that going away would be much more expensive for you too. E.g. in the past, people frequently died during such journeys. So costs were high for both parties. So given that the costs of breaching the boundaries is lower, but so is the cost of making them, the solution is still just building them higher until it's infeasible to breach them.