Because not everyone wants to watch it in a different language.
And some series don't work in Japanese, especially comedy. The purpose of the subs is to explain to us what is going on. As such, you lose the impact of the hilarity of a scene. If the dub has done its job, the scene is hilarious to us because we are laughing our butt off from the joke which was intended by the Japanese writers. As such, the dub has done its job by retaining the intention of the creators of the show.
But whatever, there is some retarded ass **** being said in here and my brain cells are starting to hurt from it. I think my IQ is going down from it so I'm out.