The Ukrainians prevented them from leaving. This whole thing began because of the major Russian component of those two states in the Donbas. The states had wanted to reunite with Russia, and then tried to secede (supported by a popular referendum) and were not recognized or permitted to leave. The Ukrainians have been shelling and otherwise attacking them since 2014, but ramped up when Zelensky took over and in the last year, probably because he felt he had Biden’s support. I guess he took random Biden ramblings as a plan, and of course he knew how deeply compromised Biden was by his Ukrainian dealings as VP.
Many folk (not Americans) in the Southwest want to leave the US and rejoin Mexico. If you were president for a day would you allow that?