The events are meant to reward dedicated players. If you're patient and wait a while a new arceus or darkrai event will come and you can get one. Or you can trade, which seems like your best option as it appears you dont enjoy events.Those are you're options. There's also hacking. I honestly don't know what the choosing process is that makes some legendaries available in game while others are event exclusive, but it keeps the game from being oversaturated with legendaries, and as said earlier, rewards dedicated players with a cool, rare, and powerful
pokemon.