The point is....the ancient astronaut theory is as plausible theory as any for a science fiction film to explore. A story about a magic god like entity creating a garden for his creations to dwell in seems less plausible in my opinion.
The difference is, one would be classified as fantasy (see: magic), and the other as science fiction.
Plus, we aren't talking about a story that deals with the latter. We're talking about one that deals with the former, nobody is doing a movie based on the latter, yet.