Why do I see junglers with Frostfang?

Frostfang actually gives comparable stats to , but gives the gold gen, some mana regen, and the passive. The tribute generation isn't disabled by killinh monsters, meaning junglers who can quickly apply all three stacks (, for example) can get a decently good benefit out of it. (The tribute procs deal 18 bonus damage and grant 15 gold with every attack or damaging spell, which totals out to 54 extra damage and 45 extra gold you get per gank) These are often more than you would likely get if you got from a wisp. The quest helps with sticking to targets as well, and the gold gen gets you item spikes faster.

Gives good stats for AP Junglers and a ton of gold and some nice bonus damage for ganks and what not.

Never seen that and I doubt it's worth. Even if the extra income can make up for the cost at some point you are still delaying your core items.

You'd have to be able to gank very frequently and make consistent use of the stacks. So it's extremely situational and a huge gamble.

is usually used by supports who have an opponent they can poke all the time and it's not like these supports are amazingly overpowered. Now think about what that means for junglers who have to spend time farming jungle camps. Can a jungler consistently get 3 hits on an opponent every 30 seconds? Or is it more likely that he has the item stacked up without a ganking opportunity?

It's cuz you're in bronze. But the passive doesn't get turned off when you kill jungle creeps. So you get passive gold and then some when u hit people with skills. Which basically never happens. It's bad

To the original question, AP jungles would take frost fang due to the quest giving you queens tribute along with electrocute essentially gave you phase rush along side electrocute as well as being an extremely efficient item. You’re likely to see it less often because of nerfs in 8.2 tho.