Why can Oracle Spice (Glaucus) not be synthesized? It is, after all, just a chemical of some sort. It is either organic or inorganic. Organic chemistry is presumably well understood because of the centuries of genetic engineering. And if it is inorganic? Well, then, surely the problem is much simpler. (I say this as a man whose grandfather had a doctorate in chemistry but whose own chemistry study was limited only to specific neurotransmitters – so what do I know.)
So what is the problem here?
Possibly an answer can be found in Dune itself. Somewhere near the end of Dune the comment comes from one of the Navigators, I think, to the effect that Spice Liquor is ‘the best’. This implies that there is a whole family of psycho-actives. However the spice found on Arakis is the most potent in releasing these precognitive powers.
So in Ornithopter there are many chemicals that can be used for this purpose. Some are synthetic. But some are better than others. Only two are the best.