I mean, the lizardmen placed giant blowpipes on top of Stegadons and Dread Saurians, and norscans have javelin throwers on top of their War Mammoths. Why should skaven not use the same logic but for hellpit abominations?
Again, they created rat ogres that can shoot with giant ratling guns, they're probably the most creative race in the world, so why didn't they come out with some sort of bunker (i don't know the technical name) to put on top of abominations filled with various ranged weapons (be ratling guns, globadiers, warplightning cannons, mortars, etc...).
EDIT: to specify, i don't mean that the hellpit abomination go around with oversized guns. I was using the stormfiend as an example. What i mean is an hellpit abomination with skaven on top of it that shoot.