Also, the borders aren't arbitrary: there are governmental structures that have built up and settled over centuries.
Really, I think there's no reason most of the laws we argue about should even be done at the federal level. We just do because the media is national, so it seems convenient and it makes for a more firey discussion, and we forget that we don't even need to agree with the other side. We can each do what we want.
Let me turn the question around and ask why you're so determined to tell people in another state what to do?