because the oil companiesw find it cheaper to import it from countries like russia and venezuala and canada
first you have to take into the account of labor costs, if your out of the US minimum wage(if it exists) is pretty low
next you have to take into account the ease of pumping it out of the ground, in some places like saudia arabia and russia(around caspian sea region pretty much only) you can stick a straw in the ground and it will come up(not realy but its an analogy) where in the US and canada we have higher wage costs and the fact that all of our oil is under a thick layer of sheet rock that covers the east coast to the rocky mountains, and we would ahve to drill through that, and many of the oil rich places in siberia and canada have thick permefrost that needs to be drilled through as well, so its easier essentialy
now the most important factor above all..........shipping costs, has to go by boat and so forth, so shipping it half way around the world isnt the best idea, most of our oil comes from russia and south america rather than the middle east, most of our middle east oil goes to our naval bases and military bases around that region
so essentialy its cheaper, thats the main reason
but when the other nations oil reserves become exhausted i think we will satrt pumping out our oil under the sheet rock, we have more il realy, us and russia and canada, but its harder to get to
also when oil exceeds 75 dollars a barrel we can actualy melt coal and use it as gas
interesting stuff ehre this site helps
http://www.gravmag.com/oil.html