chosen one

I've been playing through the Fallout games for the first time (just finished 1, currently playing through 2) and these are also my first experience with RPGs. I didn't think about it much on my playthrough of 1, but is there any in-universe explanation for the luck of the protagonists? Both...

I played fallout 2 again recently and thought the story is a bit stupid.. why did the vault dweller go far into the north? He could've just went to shady sands.. or any other place on the map. And why exactly is Arroyo a tribal village? They don't even have guns, if some slavers or raiders...