In the Dune universe, when a laser weapons hits a shield, both are destroyed in a nuclear explosion reaction.
So instead of building nuclear weapons, wouldn’t it be easier to tie a timer and a “parachute” to a laser gun and drop it from orbit onto your enemy’s city?
That’s not their question though. Their question is about why build nukes when you can achieve the same effect with a shield and a lasgun.
They did answer the question. Anyone who tries it without some SERIOUS protection would be utterly annihilated once word got around they intentionally induced nuclear detonations. Using nukes of any kind is a major taboo in universe, everyone is able to do it but when you break the taboo nothing is stopping your enemy from repaying in kind.
You can just people do neither