Why does almost every mage build the same items ?
Mage items really don't vary. Other then the mana related item, every mage gets pretty much the same thing : You need
for the incredible Ap it gives , you need
so you can have good late game damage,
is almost always needed at some point of the game, so that leaves 1 spot (Boots take up 1 spot 2) for creativity , that ends up being either a Luden's or a Rylais, most of the time. This is unhealthy, because if all champs have about the same build, then the strongest champ is the one that can abuse of that items power the most.
Bruisery mages get
Assassin-y mages with an onhit ability get
Dot mages get
If a mage needs MR, they get
and/or
Pushing mages get 
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are probably going to remain core, but the other three items almost always depend on champion and opponents. The only way to provide more diversity would be to create alternates of
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