The developers want players to experience the levels by playing them.
Nintendo has confirmed that YouTube uploading of level creations and replays will not be present in the shipping Super Mario Maker.
Super Mario Maker producer Takashi Tezuka confirmed the move in an interview with Game Informer. When asked why the feature would not be present, Tezuka said "We thought about having YouTube uploading, but it’s not in our game. The reason for that is other than just watching movies of courses, we’d rather players play them and experience them in that way."
Super Smash Bros for Wii U had uploading of replays announced for an upcoming update on Sunday.