Not an expert, but if it were cheaper, some other company would appear which would buy from the US exporter and sell back to the US importer, avoiding oil going

Not an expert, but if it were cheaper, some other company would appear which would buy from the US exporter and sell back to the US importer, avoiding oil going outside the country. The oil markets are some of the biggest and most efficient in the world, so there must be a reason for this.