They're all level 7 in RS and all level 6 in Emerald, even though all the other Pokemon used by the triathletes are levels 15-20. Feel free to down-vote this question if you think it's stupid.
There is no answer truer than GAME FREAK logic. There is no specific reason for making the Pokémon certain levels except that GF wanted them to be that way. It just can't be helped.