You can't buy them off the bat from a petrostate. Heck, even China had to enter multi-year special negotiations with Saudi Arabia to even get a batch of oil sold in yuan terms. Gaddafi and Saddam Hussein were the two proponents of selling oil in euros, and look where that led them.
The spot market is different but that's not where most of the world's oil is sold.
> The oil states throw off huge amounts of profit and oil is such a big market you need the dollars reach to service it.
The oil states prefer dollars (and have pegged their currency to the dollar) solely because of a number of historical decisions that enabled dollar hegemony, which made the oil states peg their currency to the dollar and prefer the USD. That's changing now, but only very recently and at a snail's pace.