Ash charging up Pikachu to take on Brock is one of the reasons. Team Rocket said that Pikachu is stronger than the average Pikachu, maybe he has high IVs? He also took out a bunch of Spearow early on, so if we're using video game logic, then that would mean he got a ton of Speed EVs. I doubt that is the ultimate answer, seeing as the video games follow a different system from the games, but more than anything, Ash is the main character, so his pokemon have to win for the most part, otherwise the series would be boring. Who wants to watch a crappy trainer that loses all the time?