...Michigan is a bastion of intellect. Wolverines are extremely vicious and unpredictable. Football games are played in public places with over 100,000 people there. It doesn't take a PhD in math to add it all up and figure out why Michigan doesn't have a live mascot on the sidelines.
Even if they found a cage that was heavy-duty enough to be foolproof, imagine the clusterfuck that the PETA demonstrations outside of the stadium would cause.