While it could be done (when he has the visions during his training), it could take away something from the film. Detract from the experience, so to say. It's like with the Golden Compass*: you weren't supposed to be able to see what Lira saw, she was just supposed to explain it; I think it's the same with Luke.
*Her visions were shown, and I was just giving an example; it turned out for the worse.