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The system of priority exists so that there's only a single player (or, in 2HG, a single team) with permission to take an action like casting a spell or such.  This means both that it's not a game of reflexes (whoever says they're casting something first wins), nor a case of both players waiting for the other player to cast their spell first.

If you were to allow players to take actions while a spell is resolving, you'd create those two problematic situations, or you'd actually need to introduce a priority system within the resolving spell, which would cause the problem of determining when these priority passes should happen, as well as clogging up the whole process.

Also, all existing spells/abilities were designed with the understanding that players couldn't casts spells in the middle of resolution.  The looter ability isn't supposed to give you a useable card if you have an empty hand.

As for Panglacial Wurm , it is an interesting case, but it doesn't have the problems I mentioned.  You can only cast the Wurm while you're searching your library, and there isn't the chance of your opponent trying to cast something at that time.