Because only Smeargle can learn Sketch.
Sketch is not one of Pichu's egg moves, nor is it one of any other Pokemon's egg moves. Only Smeargle can learn it. here is the link for Pichu's egg moves: http://Pokemondb.net/pokedex/pichu
How to change your username
How to add a user avatar
Why was my post hidden?
Flags vs Downvotes
Official PokemonDb League
Type a Pokemon name, type, move or ability to see instant results.