It's a scientific theory, not an "established" fact. Scientific theories are inherently tentative(fluid), open to challenges, and discarded when a new theory with superior explanatory power presents itself. It's a bit like putting together a huge jigsaw puzzle, with many pieces missing.
Guesswork is not science.
Let's try this. What makes nothing-dirt-Termie science?
No. Guesswork is guesswork. Is it used in science? Sometimes. Like I mentioned, doing real science is like playing a jigsaw puzzle with missing pieces.
The difference with science is when they are guessing(educated or otherwise) they don't call it divine revelation.
What's the meaning of your last question?
Chemical evolution is spontaneous generation which was discredited before evolution was conceived
What makes chemical evolution science and not religion?
Even assuming that your claim is correct, and that chemical evolution is discredited. So what? It's the best information at the moment. When better evidence emerges, it might indeed turn out to be wrong replaced by a totally different mechanism. That is what makes it science.
A more clearer example. Dark matter. Nobody has seen it. But it is hypothesized because it preserves what is currently the best model of the universe so far. When something better comes up, general relativity is not infallible, and can be refined if not discarded entirely.