Why is Neeko's superior Amumu ultimate on a 90 second base cooldown?

Neeko's ultimate pop blossom is straight up a superior version of Amumu's ultimate, Curse of the sad mummy. So why is Neeko's base cooldown half of Amumu's? Amumu's ultimate roots enemies hit for 2 seconds (which means they can still use abilities and spells) while Neeko's ultimate stuns for 1.25 seconds AFTER granting her a shield. Amumu's ultimate has a 150 second cooldown at level 6. Neeko's ultimate has a 90 second cooldown at level 6 - this is 10 seconds shorter than the marksmen in the game, with the exception of rapid-fire ultimates. How come Amumu's ultimate has nearly double the cooldown period when it does the exact same thing with less damage dealt and utility provided?
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