Why did Corki even have 10% aa bonus damage?

{quoted}

To make it seem like it was a beneficial passive instead of a detrimental one.

In a typical game, splitting your damage is detrimental because all your magic or armor penetration will only apply to one half of your damage.

That would be the typical case. Which is why Corki historically fell off in the late game. But in recent games. (mainly in the pro scene.) That 50% combined with his spells, already creates a damage profile that is heavily magic based, where Void staff actually becomes a good item on him.

Also, because his passive converts his Auto's attacks into magic damage, that magic damage is also affected by sheen procs, as well as crit chance.

Compare all that to how many options we have for magic resistance, and you can see how people may think that's oppressive.