I doubt there's a critter in North America that a 12 ga slug, properly inserted, wouldn't stop. Capstick wrote of using Brennekes on Cape Buffalo, so maybe there's no real advantage in going to a larger gauge.
Maybe the term,"Wretched Excess" applies.
And, there are larger gauges. Winchester still markets an 8 ga slug for kiln guns. That's right, 8 ga.
I wonder why no one has ever developed a slug load for the MM 79 grenade launcher,after all they have buck loads, grenades, Willy Pete,flares,etc.