DUH......because they don't need to buy oil.
OH Yeah! That black stuff that comes out of the ground... Seriously though, I thought you were implying an analogous connection between Saudi-Iran as there is an important one related to the nuclear issue between Venez-Iran. No sense looking for complexity where there is none... But to your point about oil, I agree, some do have it and selling it can be very lucrative. Case in point:
...Come 'n listen to my story 'bout a man named Jed A poor mountaineer, barely kept his family fed And then one day, he was shootin' at some fooood And up through the ground come a bubblin' crude... Oil, that is, black gold, Texas tea.
Jed's family story is nearly identical to the Saudi's, just more fun to sing.