Pretty simple, when I see a guy making fire in his hands, I start to charge my magical shield. You can only interrupt charges with an attack, and you can't charge and attack with that charged attack in the same turn.

You couldn't really get away with interrupting their defense channeling with your 1 prep attack either, because they've had roughly a similar time to charge as you.

Also, defending is rarely as energy consuming as attacking, that's just the nature of attacking.