Matt McIntosh cites an insightful comment that moral rules are Simple Approximations to a Fractal World:
The picture that results is of a set of limits, or metaphorically speaking a set of fences that you are not to cross, not to trespass. It is not a set of valuations assigned to every possible thing you might do. . . . morality is a set of fences where, if you cross them, you will be violating morality and will be in the wrong, but if you do not cross them, then you are fine. . . .This also explains why the rules are easy to understand and to state, and why they have exceptions. They’re easy to understand because they need to be easily knowable by everyone. Simple rules are like straight fences. Rules aren’t actually visible, they’re in the mind and not in the physical world as actual fences. And similarly, if you were constructing invisible fences, the best sort of fence you could construct would be a straight fence, because it’s a lot easier to guess where all the different parts of an invisible straight fence are than it is to guess where all the different parts of a crazy curvy fence.