The point is that they're not really "cast" in the normal sense, and it's simply inconsistent for them to be interruptable. Also, there are many scripts where triggers/sequencers aren't there to replicate pre-battle prep. I don't think Ahrimal is complaining about the ones that do so, but the ones that fire when, for example, an AI mage has run out of stoneskins or spell protections somewhere during the battle.