Occupation takes a lot of troops and resources, that frankly Ukraine doesn't have to spare. Spitballing here, but I would assume that an occupation force for that specific region would have to be at least 50 times the size of that raiding party.

Which is the point, because while it will take Russia a lot less troops to secure those borders, then it would take for Ukraine to occupy the territory, it's still forces one side to redirect assets that could be otherwise engaged on the actual front lines.