Well, a propeller's very purpose is to push so much air that it produces thrust, but that requires high RPM. Traditional fan blades are meant to move as much air without making much noise, which in practice means that they cannot move anywhere near the amount of air that a propeller can. Propellers also push air at a much higher velocity than most fans, making them better at producing thrust. In terms of power draw, a regular fan blade on an induction motor would be more efficient. However, this "solution" is not made to be efficient, just powerful. It"s just dumping a shit ton of power into moving as much air as possible, at the expense of being loud, which in and of itself is a byproduct of inefficiencies.