They want their human slaves to live in confined environments totally isolated from the natural world and be birthed via unnatural means so that they have no concept of natural conception or the fact that there's more to the world than the closed environment they live in. They will have no concept of anything other than what they are taught to believe, which will be that those who rule over them are gods and created them for the purpose of serving them, which they will do out of ignorance and necessity. They won't even know about death, so that they won't know that they can escape the hell they were born into.