Spice was largely unknown until people started harvesting it on Arrakis. It was only on Arrakis that the sand trout had a life cycle that produced spice in usable quantities. And even then, no one really cared about it until the Butlerian Jihad and the empire needed an alternative to AI for FTL navigation.

As much as genetic manipulation (the bene gesserit breeding program) and advanced biological technology (the tleilaxu) exist in this setting, I never got the implication that the sand worms were genetically engineered, let alone to produce spice. In fact, as important as spice is, it took a hell of a long time (thousands of years after the events of Dune, in an interstellar empire which already existed for thousands of years at that point) before an artificial substitute was created, and the worms themselves were never successfully transplanted to another biosphere (which you'd think would include genetic engineering if they couldn't survive naturally).

Some of that is, of course, Leto's influence in controlling the spice, but that's why I point out the empire was quite active before then. It was in everyone's interest, except the emperor's, for spice production to be disseminated, and it strains belief that the imperial house was able to stamp out every attempt to produce spice elsewhere.