"Mana items are bad on garen"

if the enemy team doesn't have any tank is the good path to go

but why tf would you build this on garen ?

No idea why even bother with triforce on Garen, he gets little benefit from it when compared to everything else he could build for damage. Also if the enemy is full of DoT champions and constant magic damage from the same source do you really care if you get nothing out of the mana when the passive is more than making up for it?