What Ecc2ca said. It's just part of the rules.
In earlier editions there was a (flawed) rationale for it: spellcasting requires extensive somatic gestures that are difficult to produce if you are encased in steel. 3e allowed you take feats that allow you to cast only with your voice (or only with your body).
Essentially you are doing a little dance but can't dance in those boots.
Last edited by KillerRabbit; 19/02/24 07:18 PM.