Why is it okay for Volibear to deal Assassin-Level damage while he buys full tank items?
Assassin level damage isn't a thing in LoL, that's more of an MMORPG troupe where rouges do the most damage.
In LoL, there are 4 main DPS classes that all do roughly the same amount of damage as one-another in Fighters, Assassins, Marksman, and Mages.
Were they mostly differ is there means to survive long enough to deal damage.
Which is almost pure defense for juggernauts, a mix of defense and speed for the bulk of the fighter class, speed for assassins, a mix of speed and range for most marksman, and either a heavy focus on range or a split focus between range and defense for mages.
As a Juggernaut, Volibear does as much damage as other kill-based classes and is pretty tanky.
Given the shinnaingans
can pull while they do tons of damage and remain untouchable, I'm not sure why you'd feel there's a problem with Volibear being tanky and doing the same level of damage.
Sure, sucks when he's carrying his team and you're on the recieving end, but that's the nature of any fighter, assassin, marksman, or mage carrying their team.
He gets to have godly innate dive potential
You have convinced me you have no idea what constitutes as good or bad diving potential.
Briskly walking towards opponents while threatening to bear-hug them is not godly dive potential.