We can't. Domestic oil is light sweet crude, but our refineries are designed for heavier sour crude. So we have to export our domestically-produced oil in order to import the oil that are refineries are designed to process. If we attempt to process our domestic oil through our domestic refineries, production drops significantly due to the wrong equipment in place, and supply would not keep up with demand.
Seems kind of short-sighted on our part, but I have yet for an administration to ask my opinion.