I feel like you must be a newer player. FG Ezrael has been a thing since the item was introduced, popularized with the "Blue build" Ez, which was a kite and poke focused build that used Muramana, IG, SotLE (now removed) and LW to deal good damage from an incredibly safe distance. Since then, this build has been nerfed and IMO, this item remains balanced on Ezrael for a variety of reasons.
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It's not a purely offensive item. CDR, Mana, and Armor aren't wasted stats on Ezrael by any means, but they also aren't the pure AD or offensive multiplication that ADCs usually thrive on.
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It takes the space of Triforce. Normally, this would be a good idea, especially in a faster meta, but you end up losing out on some incredibly strong passives and an absolute pile of stats that benefit Ezrael greatly. IG is certainly the better early to midgame item, but come lategame, that advantage drops off in Trinity Force's favor.
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Ezrael's "weakness" of lacking waveclear is a relatively common one for ADCs and one that tends to disappear anyway as the game goes on. The fact that IG assists him to clear tends to become less and less important as the game goes on, not even considering the fact that it's not a huge advantage in the first place.
That all being said, it's certainly not a bad item on Ez and if you think it's OP, go out there and abuse the heck out of it. Ez has always been one of its best users and the item is traditionally not considered out of place on him.