I always taught it like this:
A runner is out when hit by a batted ball. Period.
Except, that's not fair to the runner when the ball goes through or *immediately past* a fielder who is right in front of the runner. The runner thinks the fielder is going to glove the ball, and the runner has no time to react when the fielder doesn't. So, we have an exception to the rule.
Except, that's not fair to the defense when another fielder could make a play on the ball. Heck , the first fielder might let it go on purpose because the second fielder is moving in the direction of an eventual throw. So, we have an exception to the exception.
And, now we have the whole rule.