Herbert needs the Butlerian Prescriptions as a plot device, to make the story focus on humans and humanity as a whole, instead of technology.
It is simply considered unethical to build machines that resemble humans in appearance or thinking, because it is considered to be derogatory to humans and because thinking machines will be the doom of humankind. It has therefore become a cultural taboo to construct them.
The political system is balanced in a way so that no single political entity, neither the Landsraad, nor the Emperor or a single great House, can grow more powerful than the rest combined. Circumventing the dependence on the spice (which we know, controls all of the universe), by building thinking machines for computing and space travel, would allow such a growth in power.
A world outside the Imperium becomes technologically advanced.
I'm pretty sure that the Padishah Empire encompasses the entire known universe (all space settled by man). The closest thing to outside worlds are probably the technologically advanced fringe worlds Ix and Tleilax, where they in fact work close to the limits set by the Butlerian Prescriptions.
But let's consider what would probably happen if some political faction in the empire tried to reintroduce thinking machines. All other factions would immediately turn against them. They won't let anyone get away with something which is basically a crime against humanity and a serious threat to their very existence.