For the same reason that powdered-sugar-and-water “five finger” icings taste floury and chalky: uncooked cornstarch. Powdered sugar is about 3% cornstarch, which keeps it from clumping up into a solid mass in the bag. Normally, because powdered sugar is used as a garnish, dusted onto cookies, waffles and the like, we don’t notice it. However when it’s served to us in the form of an icing, we do. That’s why, given a choice, I’ll take a thin glaze of poured fondant over a 1/4-inch thick layer of rolled fondant every time. Of course if you look at a layer of rolled fondant as a “peel”, sort of like the peel of an orange or a banana, it becomes less offensive. That’s generally how I treat it at weddings, or even when I make a rolled fondant cake myself. All rolled fondant cakes have a thin layer of buttercream just beneath their skin, which is where the real money is, at least for me. The fondant I’ll happily push to the side of the plate.