There are pragmatic reasons beyond the practical ones. Don't forget the political and economic ecology of the empire. It runs on a system of hydraulic despotism. If there were more than a single, well defended font of the vital resource, House Corrino couldn't measure out the release to enforce its power. Think of an upriver nation damming water to keep downriver nations under their thumb.
You might think of a worm transplant as building new factories, distributing production, but its more like OPEC owning ALL the oil fields and then creating renewable oil fields in less secure territory. And the Guild gets to have power over it in transit. Upsetting the balance could prompt the overthrow of House Corrino by another Major House.
Corrino assumed the harsh nature of Arrakis and mutual dependence on the Guild would prevent the loss of their singular font of power. No army could land (unless by Guild conspiracy), let alone carry out an effective assault. A second source would put Corrino in danger of assault on two fronts, possibly by the whole Landsraad if they can even work together, maybe even with authorization to Nuke the Emperor's secure source or his Sardukar training prison planet to dismantle his defenses. One thing clear in the Dune universe is that seat of power is precarious, and any major change introduces unknowns ripe for manipulation and power grabs.
Had someone figured out how to do it, the transplant may be proof of principle for another, and another, and pretty soon each faction has their own Spice source and is no longer dependent on CHOAM and the Empire's hand on the valve. The balance of power is broken. Chaos ensues. Nobody wants large-scale war if it can be avoided, even Paul. The Guild would probably win in the end, because they have the only Navigators (the new secured source), checked by Landsraad nukes. Until someone else invents Navigation, like the Ixian technophiles eventually do, and then the chaos repeats.
Hydraulic despotism is precisely the means by which Leto 2 "preyed" on galactic civilization, enforcing peace to guide the evolution of genetic and cultural traits that would resist the totalitarian despotism of prescient beings (i.e. future God Emperors and K. Haderachs). He weaned the galaxy off its spice dependence - and again this was only possible because he held the single source, the only font of galactic power. He even shrunk the source with his terra-forming, confining the worms to a smaller range and bringing spice production to a trickle. It's like reducing oil reserves, everywhere, abruptly and letting post-peak oil play out: famine and diaspora and totalitarian control over its course.
So if the God Emperor's Golden Path was always Herbert's end-game, for plot reasons it was necessary to keep the single, well-secured source in place. If it were possible before Leto 2's modifications, we'd end up with a very different story that wouldn't explore the hydro-despotic themes as thoroughly. Nor would he have the necessary grip on the Bene Gesserit to prepare them, culturally, for the Honored Matres threat and their own spice source after his death.