The movie was about causal loops from the start (HEAVILY hinted to with the "ghost"), so the answer was more like, "because that's how causality works, and we are basically observers to the whole thing," as opposed to "advanced humans."

They mentioned that they were attempting to conserve as much gas as possible. A traditional booster allows them not to waste the gas in escaping Earth's gravity. It's also why Coop landed on the first planet entirely on angle.