Why doesn't Ash Ketchum catch legendary Pokémon? In the show, Ash Ketchum hasn't caught any legendary Pokémon and he loses a lot of battles and people say that he is a pathetic trainer. Why does he not catch legendary Pokémon to expand his arsenal?
There are several reasons to that:


It's a TV show, you wouldn't watch a TV show where the hero is absolutely the strongest and no one has a chance of defeating him, right? Think of what would happen should Ash have caught Dialga, Palkia or Arceus.
It disrupts "the balance", it was portrayed in the anime that most legendary pokemon serve a role as some sort of "god" of a force of nature, (Kyogre of the Rain, Palkia of space, etc). If you capture them, you might disrupt the balance by "not letting them do their jobs".


Those are the two excuses I know.