It would make Gragas too weak. By building AD Gragas, both Q & R just seems to be just plain CC with no extra damage, plus the fact that his E still needs to deal damage, and making it AD just turns it useless, reminding you that it would stop on any enemy units, even minions & monsters, which limits his mobility. I know that his W deals %HP damage like Fizz does, but would you only want to do it once every 8-4 seconds?
About Fizz, he is a more of a DPS kind of fighter champ compared to Gragas. At least with the
builds, he would dish out tons of damage with a couple of autoattacks, plus with his W that deals %Hp damage over time. Unlike Gragas whose only limiting mobility is his E, Fizz E would actually be more convenient in terms of mobility & juking. Fizz's Q also deals on-hit damage, which applies lifesteal, & the %Hp damage from his
.
He also has a shark which latches onto a single enemy, slowing and then knocking up & then receives 20% amplified additional damage from Fizz, while all AD Gragas's ult does is exploding the carries further away with no damage.