What if the Oathgates don't like to Urithiru, but to something in urithiru?
That is, imagine there's some huge, immensely complex fabrial that powers the gateways. The exit end of an Oathgate can be anywhere, but the entrance end has to be at the Tower of Gates (random name i just made up).
The idea being that all the power for the gate came from one end. Thus, a gate can only be opened connecting the Tower to somewhere else, and they didn't build another Tower because it was way too much expense and trouble for very little gain.