Probably to make Luke all mysterious, (has he fallen to the dark side already?) and build up to his arrival at Jabba's palace. Threepio standing outside the cave could have been matted into a cave set shot later.
Don't forget, Leia and Lando make their entrances in disguise too.
Doubles are used in films all the time. You don't pay Harrison Ford to walk down the street with his back to the camera, or for an insert shot of pushing a button!