Why can't bosses/monster hit you more then once?

To change invincibility frames will screw up many players. I myself used to use it to bypass Bigfoot. I also use it to take a small hit so I'd avoid a larger one (touching a Firebrand for instance so I don't get smacked by an Elder Wraith). This kind of stuff would benefit Warriors like hell. 15% chance to stun an enemy per hit would really add up if characters can take multiple hits in a second.