It's because we're also the biggest refiner of oil, and the majority of our refineries are designed for "heavy" crude that is native to most countries (Saudi Arabia, Venezuela, Mexico, Canada).
Our oil is "light" crude and most of our refineries can't handle it unless it's mixed with heavy crude.
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The problem is that not all crude is created equally. While the United States is now the world's leading producer of oil thanks to the shale boom, most of those barrels are very light. That high-quality crude is a poor match for the Gulf Coast's decades-old refinery system. Shale oil is blended with heavier barrels to allow refiners to maximize production.
US refiners used to rely on Venezuela for those heavy barrels. However, US oil imports from Venezuela have vanished because of the Trump administration's sanctions on PDVSA. The United States imported zero barrels of oil from Venezuela last week, down from 517,000 barrels per day a year ago, according to the Energy Information Administration.
Normally, the United States would turn to Saudi Arabia, which also produces heavy barrels. But Saudi Arabia has slashed shipments to the United States as part of OPEC's effort to boost prices.
Canada, another major producer of heavy crude, is already the leading source of foreign oil into the United States. And a shortage of pipelines from Canada will make it hard to send much more.
"There are very few alternatives in the short-term," Ryan Fitzmaurice, energy strategist at Rabobank, wrote in a note to clients.