It is a truth universally acknowledged that Man cannot live on Bread alone. Some may claim that the other thing needed is something spiritual or otherwise incorporeal, but we claim it to be fruit or custard, some butter, and sugar.
Thus, one can plainly see that pie is simply a continuation of the One True Goodness, bread. Make a loaf with sugar and egg, flatten it down, and fill it with fruit or custard or something other nice, bake, and eat.
The original Bread Bowl was, in fact, a pie.