Earth or the relative vacuum of interstellar space isn’t really the issue. Think of it this way, there has to be energy to extract and a mechanism for converting it to do work. If you want to move something you need something to move it. Nothing comes from nothing, as it were. If you want to drive a vehicle between stars, you need an energy source to drive it.
I think sometimes the confusion derives from what motion really is. You wouldn’t expect a cause without an effect, would you? On a really basic level, the conservation of motion is the logic of causality.
