why are cut cards shiny on one side and not on the other side?
Has to do with how each side is used. You want to always have the shiny (smooth) side facing the bottom of the deck, and the non-shiny (textured side) facing your hand. That way, the texture of the non-shiny side makes slippage less likely, while the smotthness of the shiny side does nothing to damage the "face" of the bottom card in the stub.
You're welcome . . .