If you take into account that they are not taking EVERYONE on the planet as slaves, you realize that they are indeed investing in re-education of population at some level. But when you have massive populations, it can in a way pay off to haul off large portion of potentially rebellious conquered populace elsewhere, to planet they are unfamiliar with and put them to slave labour which ensures you get something out of them while malnourishment and disease reduce their numbers. From this point of view it is very functional long term policy. Reduce conquered population to more manageable level and get something out of displaced population before they vanish into afterlife.