The trainers can call back pokemon, but only from a cerain distance. They can't do it if they are too far away from their pokemon. They would also need to aim pretty well to get them-a difficult task since the steal usually happens pretty fast. They can miss (see Ash's Krabby in his Kanto League battle. The main reason they don't do it though is for dramatic effect; what would be the point the whole stealing thing if they could get their pokemon back that easily?