Oil and Oil-derived products like gasoline have a finite life after being pumped from the ground.
If you are producing (refining) petroleum products, you have to sell those products within a relatively short period of time ... for gasoline it's about 90 days, although you could probably get away with 120 days. Other products have different lifespans, but they all have limited lifespans ...for motor oil it has to be in a container within a specific time period, whereupon it can have a much longer shelf life. But that means you need to sell it to an oil company relatively soon after it comes out of the ground.
The old business method was predicated on leaving it in the ground, because it was believed that it would be "good forever". That way of thinking is changing; producers are beginning to fear they won't be able to sell that oil into the market in 20 or 30 years time. So they are pumping it instead.
The North American oil market is controlled by independent producers who own wells or well rights. Corporate owned wells are only about 30% of the market. So it's that 70% who determine how much wellhead oil is available at any given moment. These are basically landowners, mostly farmers and ranchers, with wells on their land. They decide how much oil is available. There is a self-controlling mechanism whereby they tend to close wells when prices are low and open wells when prices are high. Oil prices are relatively high now, so they are pumping more oil from the ground (and that is why the world price doesn't always reflect the pump price; for example when the world price is low, the supply is also low which drives up the pump price).
Take the above two factors together, and you have a large supply of perishable product, so it gets sold to markets outside the US when the total exceeds America's ability to use it in the short term.