Clearly, your comment has no premise and is therefore an unsupported opinion.
What you are saying is that items change the power level of Champions?
Yes, you are right. However you use this information to infer Volibear's OP? This is wrong.
Allow me to explain.
Every champ is balanced. Imagine every champ has the same base stats and abilities and skills....
A champion's base stats, abilities and skills are given gold values, the same gold values that are used to calculate an item's gold cost/ build cost. Those base stats are all calculated to "start off" (naturally is a better word) balanced.
Like Scaling runes vs flat runes, cost difference in the values between late game vs early game are also taken into consideration when balancing a champion.
So far that is how all champions are "Naturally balanced"
The only time you'll see this natural balance become unbalanced is through the playing of the game.
Farm, kills, assists, objectives, all translate into Gold, which is what makes the players and their champions shift from this balance. If your opponent has more gold, he will benefit more from the items, which are available to all champions equally.
This extra power from items, which are a result from how well you're playing the game, will further highlight the naturally balanced power of your champion. This is only logical, as you'll only be extra powerful from items, compared to your opponent, if you're playing the game extra better* than he is. And these items further compliment the natural power spikes of a champion, which are naturally balanced and already incorporated into the gold value that justifies it.
For example: If a Lee sin gets fed in the early game, the item difference then, will have a more drastic impact on the game, than if he was to get fed late game. Why? Because Lee Sin's calculated natural balance base stats takes this into account, this shift of better early game performance, and his late game performance is balanced accordingly.
The game rewards you for being better than your opponent at the game, that's how you win the game. This is why I've said before that league is a damage output game. You do more damage by staying ahead in gold.