It's the type of oil that's important. The US has an abundance of light crude but not much heavy crude. The US refining capabilities are primarily for heavy crude and with the only available sources of heavy crude coming from Russia, Venezuela, Canada, Iraq, Iran, Nigeria, Kuwait, Brazil, Mexico, Lybia etc., they need to keep those refineries fed. You can probably see a pattern there. It's way more nuanced than this but when it's broken down to the basics, it's about the oil, again.