Because it's enhancing botrk and some on-hit abilities and god knows botrk is far more broken than nashor's tooth.
- you don't need any other stat to make it work (like you kinda do with AP and Nashor's).
Let's compare the damage of botrk and Nashor's tooth
against a 1500 current health target, which isn't that high :
botrk grants 1500*0.08= 120 damage on hit
Nashor's tooth grants 15(+.15 AP) damage on hit
You need 105/.15 = 700 AP to make it deal as much damage
Yet, good factor for Nashor's, MR is more expensive and armor growth/lvl of champions is stronger than MR growth/lvl of champions. So the fact it's magic damage makes it slightly more valuable.
Yeah, but botrk gives lifesteal and the active as well.
Fine, Nashor's give cdr.
Overall, Botrk is still stronger as an item.
Now devourer. It multiplies by 1.5 the damage of on-hit effects. So basically that botrk will become even more effective. Or Nashor's tooth actually. Devourer + Nashor's tooth is pretty common on
jungle.
And it also multiplies by 1.5 some on-hits that are already very strong (Yi's E, Shyvana's E)
I'm not sure if you already played a very tanky champion against the likes of Master Yi, but this is how the death recap looks like, approximately :
1200 true damage
1000 botrk damage
900 basic attack
Now that might be somehow understandable if his whole build was based on botrk (the true damage is still beyond stupid) but when you actually look at his items and see he has
and that even with the last 3 items he doesn't deal more damage with crits than the botrk proc, or the true damage, for that sake, you know there's a problem.