Its mainly because it makes no logical sense. You van just put the sword in the shield hand then cast the spell it would not even be that hard.
It’s almost impossible to do that in a high stress environment without dropping the weapon, and if the shield is a buckler or a viking shield it’s completely impossible due to how you hold the shield. In any case you make the shield unfit for blocking while holding the sword.
If you want to throw spells you should do as the romans with a small sword with a scabbard on the sword arm side and sheathe the sword to free the hand
While i dont have one of those shields irl I can easily hold 3 swords in one hand so I disagree. Besides if your opponent is just letting you do hand movements eitherway being able to block is irrelevant. Not to mention you can also hold the sword in the palm of the hand by facing the palm uppwards.
