The simplest answer is: if the price in, say, the EU is higher than in the US, there's an incentive for someone pumping oil in Texas to sell it in Europe rather

The simplest answer is: if the price in, say, the EU is higher than in the US, there's an incentive for someone pumping oil in Texas to sell it in Europe rather than to a refinery in the US. If the price in the US rises to the same as EU (net of shipping) you might as well just sell it here.