Gym leaders mass breed there pokemon, they have their team and a league approved team. Brock is relatively weak with his league sanctioned team to give people hope and convince them to start pokemon training. In the cases of levels they stuff rare candies down the babies throats. For happiness evolutions they also stuff the ev reducing berries down their throats. (Also note that higher level gym leaders lose very rarely) The league started killing pokemon so that they could drum up more views, and pay more for advertisement. Which is why Illven quit in protest.