Those people will still need infrastructure and housing and workplaces that have been destroyed by the Russian Army.
You might need a lot of infrastructure and housing to move in a lot of population, but you don’t need all that many people to drill a bunch of oil and gas wells, and send a pipeline back to Russia. For those who have not been paying attention, Putin became concerned when oil majors signed exploration papers in 2012, which directly motivated Putin’s move on Crimea in 2014 which promptly led to canceling of those papers. Putin’s goal is both regaining the Soviet Union AND grabbing all the resources he can. He does not have to be tricked into doing what he wants to do anyway. His ego prevents him from imagining he will fail or destroy his army in the process.