It does serve their interests - and Iraq's interests, too, I might add.
Oliver, the economic advantages would have to be large enough to justify a big-honking expensive war with thousands of dead American soldiers. Not to mention Iraqi deaths and alienating the international community. As far as I know Iraq still hasn't reached the level of oil extraction it had before the war. It's like Silverstein's fantastic insurance scam that loses him money. Now you're probably going to say that the war effort didn't work out as the US thought it would. But it would have meant an expensive occupation even if everything had worked out as planned. If you want to show that the US anticipated a monetary net win larger than zero, you're invited to do the math.
And even if you could prove that the US was dumb enough to think they would make money out of this war, why do you believe they would repeat the same mistake with Iran? After all they already got burned once, public opinion is against more military adventures, and Iran is a much tougher enemy than Iraq was. It doesn't add up.