You can't cast a spell that copies another spell while something with split second is on the stack, but there are other ways around it
(b) remove, store and further insure the Lot at the expense of the Buyer
Hundreds of cards, in fact

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The combination of these two factors means Pachirisu Ex can consistently out-aggro decks reliant on evolution, taking out their Stage 1s and Basics before they can reach full potency
After consulting with some smart people, I came around to understand that its for power level/game play reasons (as opposed to flavor)