The US maintains the trade of oil for dollars at the point of a gun.

They do this because it allows the US to create a perpetual demand for dollars worldwide. That demand maintains the value of the currency.

This allows for unequal exchange, where the US can purchase goods from around the world for significantly less than they are worth, maintaining poverty and thus cheap labor around the world.

It also simultaneously allows the US to maintain trade deficits without losing the value of its currency. Thus Americans can consume multiple times what other people around the world consume without having to actually produce it. It also allows US people to have an advantage in any transaction abroad.

All currency is convertible to another currency. The arguments that somehow accounting for fluctuating currencies makes the system easier ignores that the US dollar fluctuates in value just as every other currency does. The real reason for this arrangement is that it is a major part of US economic hegemony.

When the US controls oil via its financial systems and enforces this rule militarily, the US becomes, in a sense, the de facto ruler of the world as oil is a necessary resource for every modern economy.

The reason we are starting to see this break down is because the Chinese have shifted the world's economic center of gravity eastward while alternative forms of energy are becoming more and more viable. Oil will have to be abandoned for climate change eventually, so it makes sense for other countries to start pushing back now, rather than waiting for later.

It should be clear that the USD being the forced currency for oil is a great arrangement for the US and a shitty one for everyone else.