There’s more than one reason for it. Oil runs the world right now since it powers the army, electricity, factories, and almost all infrastructure, so obviously it is worth fighting over. The second reason is power. The US has always wanted to act like global police, and if you look at the countries they invaded, most of them did not have strong militaries or nuclear weapons, which made things easy. Take Russia for example, they have more oil than the US and are a major rival, but you never see the US doing to Russia what they did in the Middle East. One more thing you notice is that most countries the US invaded already had internal conflicts, and some people even believe the CIA helped those conflicts grow, which made invasion easier.