No one's been able to find mass teleportation spells that can go across factions, otherwise they would've already been using them. There are only two times people have shown that kind of power, the two randos who rescued everyone a few years back, and the cult. Since we don't know more about Oliver and Elizabeth, but we know the cult used High Magic, so if we can copy that, great.

My reason is so people can leave factions if they want to and escape to wherever they want to go.