Well I got that wrong. I misunderstood, I never watched the XY anime :P.
Well, if they made Pikachu and all the others super powerful, then there would be no challenge. Ash would just rock up and beat the living daylights out of any trainer. There would be no build up, suspense or challenge. People would get bored of Ask winning EVERYTHING up to maybe E4, and then those matches would be quick.
In terms of game, probably just a little to hard to code, that or we don't want a few Pokemon getting massively overpowered.