A: Because someone breaks them.
Q: But... but...
This statement is not true of all laws. In today's America, it's possible to write and pass a law concerning behavior that's pretty well theoretical, which is a reflection not of how enlightened we are but of how bored legislators must get.
The laws that matter most, the laws that create society's framework and keep its many movement parts oiled and functional - these laws reflect an ethical state that already exists. They are in, at best, the fat part of the curve of societal evolution rather than at its leading edge; they may even be at the tail of the curve. What they are not is immutable.
Belmont Club discusses the changeable nature of law in the context of a tremendously powerful challenge to international law:
[UK Secretary of State for Defence John] Reid's points taken together comprehensively call into question the international constitutional system. It is unlikely the issues raised by those questions will be resolved any time soon because those issues are typically addressed by the victors after a war (Utrecht, Westphalia, Vienna, Versailles, etc) to codify a consensus that has emerged in the course of events. All one can say with the conflict still in progress is that current concepts of the Rules of War, pre-emption and territorial sovereignty will be called into question; that they will change under the pressure of future events is all but certain; but what they will change into is anybody's guess.
The points to which Wretchard refers are (1) whether the Geneva Conventions need to be updated in order to reflect an enemy that does not make war by the rules that the Conventions took as given when they were developed; (2) when pre-emptive military action against another country ought to be permissible (at present, legality for this type of action exists in a fuzzy cloud of "imminence"); and (3) the nature of sovereignty, in terms of whether a national leader, acting entirely within his own borders against people entirely under his leadership or rule, can act with impunity against them.
These three points are already settled among American hawks. Obviously Geneva has been no help in dealing with un-uniformed, extra-military, non-state actors; the remaining question is, do we rewrite Geneva, or do we somehow convince those who believe they do apply to these shadow-enemies that they don't? Because the arguments for the latter have been made and re-made without notable success so far, a rewrite would appear to be in order. Second, when small and unacknowledged groups can inflict mass casualties, pre-emption becomes not just a choice but a responsibility of a nation's leader. And third, it's astounding that in a world where Holocaust museums are thick on the ground, there's any question that the world community legally can and morally should "interfere" with a sovereign (elected or not) who is pursuing genocide or other obvious outrages against human rights.
But these same three points remain up for debate among too many others. I can't figure out why; they appear self-evident from my seat at the keyboard. I've long believed that "international law" was a lovely castle in the air; and the events of the last decade or so have confirmed my belief again and again. No one who understands power abides by international law unless doing so somehow consolidates his power. American hegemony is dangerous - of course it's dangerous; hegemony by anyone would be dangerous, because it implies, if not power, then influence beyond the realm of "fairness," and as such is open to abuse. But power, like nature, abhors a vacuum, so in the absence of multipolarity of powers there will be a hegemon; there is, in fact, a tacit hegemon, and it is the United States. The check on our hegemony is partly economic, partly political. But increasingly it appears that Europe sees its peril and is coming to accept a form of clienthood, in that Euro-armies are more and more involved in Afghanistan (how can you learn to fight an enemy if you never fight that enemy?). What Asia will do is beyond me; so far they appear to believe, or to act as if they believe, that their societies are too closed to be vulnerable to Islamist extremism. Africa is part of the problem at present, where it figures at all.
It's going to be an interesting century. New American? So far, yes.