Artificial limits on certain game mechanics

Wondering why there are artificial limits on certain game mechanics such as ability cooldown (40% / 45%) and attack speed (2.5). I understand that champion balance is a factor, but if you're willing to commit resources for a 90% cooldown Veigar ult, it should have an itemization tradeoff which limits mana or damage. In other words, why can't these artificial limits be built into itemization to simplify champion/item balance?
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