Volibear is very deceiving throughout most of the game in terms of power
You think you can just kill him at level 2 and don't have ignite?
Level 4? Time to fight voli without my jungler!
It's 3 minutes and 25 seconds into the game? Better stick around in my lane while jungle voli goes back to get his jungle item and heal up.
Level 6? His ult doesn't deal that much damage.
No way his passive will save him. I'll just dive him under tower.
Rip in many bite-sized pepperonis.
Edit: For everyone who says he falls off late game-
.
He wants the booty.

specialy omen which is the best against voli

who actually can get to your carries without taking hits, and who are either tanky as fk while dealing way more damage than Voli and have hard CC to boot, or are slippery as fk and kill carries in a few autos?
+ 
(Although Talisman really isn't a cost efficient item tbh because it's the only support item that provides NO MEANINGFUL combat stats)