Indeed it would make his ult do more damage but it would take away from damage output from both void ooze and caustic spittle. So the AD would make you do less in those departments. So i'd gain maybe a bigger nuke for ult but subtract from kog'maws other 2 nukes and i'd rather have 3 good nukes rather than 1 good nuke and 2 decent nukes.

Why not go for Hybrid so that Living Artillery gets a boost in both departments? It's modified by both attack and magic damage. It would be even more of a nuke if you put something in both.