Because all the player characters are on the same level of prodigy, resulting in near identical evolution rates and moves learned in training. Other trainers often aren't so good, like that fisher with 6 lvl 27 Magikarp. On the flipside, some trainers and wild pokemon may even exceed the PC's training ability, resulting in this.
Of course, there's other possible reasons, like being placed in an evolve or die environment.
