Shipping: no easy to ship from producing site to say NE coast, so they might buy from Sweden and Saudi Arabia, while producer, say in Texas might have a pipeline to the west coast and export to China.
Price/contracts: some large buyer (say refineries) buy on long term contracts and might still buy at prices negotiated when the US had policies restricting domestic production. Also, not everybody sells at market price, so you could buy foreign for cheaper than what's currently available domestically.