Hmm... Wonder what'd happen if a trainer tried to get there themselves. Could always make a makeshift boat and use water Pokémon to sail. Or fly, but without a map...

[He trails off. There's a ton of potential for disaster. Getting lost at sea without help. Getting caught up in some ocean storm...]

That might be it, though. Pokémon have been working with humans for such a long time that they're probably just conditioned to trust them. That, or Pokéballs brainwash them, but I doubt the people here'd create something that twisted.