pantheon shield passive suggestion

suggested innate: Each basic attack from champions, big monsters and big minions grands stacks. At N stacks pantheon blocks next basic attack

i dont understand why pantheon has to attack and cast spells to be able to block attacks. Why not every N attack blocked so when 5 champions attack him he would block much more attacks rather than if he cast spells and lands basic attacks to active the his block shield. This also makes him very frustrating early game and some people say his innate is really weak late game