Why is Galio's ult on a higher cooldown than Amumu's?

Galio's ult is on a 5 second higher cooldown than Amumu's at rank 2, and a 10 second higher cooldown at rank 3. I'm here to ask why.

• Galio's ult deals considerably less damage if Amumu builds full AP.
• Galio actually needs to be able to sustain many hits to be able to ult.
• Champs like Leona, Garen, Udyr, and Xin Zhao can cancel his ult while taunted because of their CC applying auto attacks.

Riot, this champion needs a few buffs. Why not start with this:

• Galio - Idol of Durand (R) Cooldown reduced to 120/110/100 from 150/135/120