How had I never heard of slab pie?

Smitten Kitchen--a favorite blog--this week put up a recipe for apple slab pie, which is an apple pie baked flat in a jelly-roll pan. I'd never seen such a thing and found upon googling it that apparently this incarnation of pie is quite a common way to make pie for a crowd and appears to be a midwestern specialty. I bake, I live in Pittsburgh, which is almost in the midwest. How have I missed this thing?

I think I'm going to try making slab pie for Thanksgiving.