Meat-based pies are simply a food substitute sorta like Kraft American cheese-food slices.
This doesn't even make sense. Neither Shepherd's nor Pot are "food substitues" as you say. Additionally, Australia has a version of a meat pie, which is considerably less common in the United States (but available if you know where to look)
And who are you kidding with meat cupcakes. There is no such thing.
1) there is such a thing, and apparently I'm not the only one that has made them (although I iced mine with Mashed potatoes):http://www.dauwalder.net/MeatCupcakes/
and there has also been a storied meat cake:
http://www.blackwidowbakery.com/demo/meatcake/but as I said they're novelty, not common dishes.
Pies are mostly filling. Cakes are frosted with icings that melt in your mouth. Pies are the default only when there is no cake to eat. CAKES RULE! Think of it like this: if a PC were a pie, and a cake were a Mac, why would anyone want a pie?
Pies offer, when done right, a savory element to contrast the sweet filling. Cakes on the other hand, are simply layers of sweet, there's no complexity afforded to cake eaters.