Who would you rather get to "keep the oil"? The country that sunk hundreds of billions into first destroying and then rebuilding Iraq, or some other country?
But even assuming that the USA should "keep the oil" -- how would that work in practice? You can't just pump it all out and make a quick exit. Estimates say that there is 140 billion barrels of the stuff. (That's over 22 cubic kilometres!)
The United States would have to maintain an occupation force to protect the wells and pipelines for years while they're enjoying the "victor's spoils", as Trump puts it.
It's unlikely that any other country would purchase oil produced under such circumstances, so the oil would also have to be shipped to America... And the only shipping route out of Iraq goes right under Iran's nose. You can see how this might escalate quickly.
Trump's idea of keeping Iraq's oil would make for an interesting plot point in an alternate history fiction about World War III.