Why is full tank Olaf allowed to deal bruiser-level damage?

Obviously you just fed him to the point of senselessness.

Tanks are threats primarily due to their utility. Examples include Nautilus, Sejuani, Gragas and Maokai.

Fighters (i.e. Bruisers, Juggernauts) are threats primarily due to their damage. Examples include Dr. Mundo, Olaf, Darius and Garen.

Even if Olaf builds nothing but defensive item, he is still a fighter. He doesn't suddenly get enough utility in his kit to be considered a tank.

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If you go full tank, you're no longer a bruiser and your damage should suffer. Olaf gains nothing from going for a bruiser build since he keeps pretty much all his damage when going full tank, so why bother? Obviously he's in need of fixing.

What makes this obvious?

He's picked at the LCS level for his CC immunity, not because his damage is overwhelming.

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If you go full tank, you're no longer a bruiser and your damage should suffer. Olaf gains nothing from going for a bruiser build since he keeps pretty much all his damage when going full tank, so why bother? Obviously he's in need of fixing.

Have you tried playing Olaf? Try going mostly damage on him and see how far it gets you. He's literally a shadow of what he used to be.