I think the easy explanation is that the enemy also has one or more 18th level casters who could teleport anywhere. Thus, the PCs' nation keeps their highest level casters at strategic locations in case an enemy caster pops in. The enemy does likewise.

If both sides have high-level wizards, then it's similar to having nukes. They can hit anywhere on earth, probably can't be stopped, and unleash untold damage - but revenge is guaranteed. MAD is the answer to your question.

ap