The US government's policy of "encouraging" countries to sell their crude in USD (in a "nice country you've got there, shame if something happened to it" way) keeps them in effective control of the price of oil. The last time someone tried to sell in Euros instead [0], the US DOD spent more than $1,000,000,000,000 [1] proving their willingness to back up that threat.

[0] https://www.theguardian.com/business/2003/feb/16/iraq.theeur...

[1] https://en.wikipedia.org/wiki/Financial_cost_of_the_Iraq_War

https://en.wikipedia.org/wiki/United_States_Fifth_Fleet