The problem is the other side:

The seller has a huge interest to not deal with too much fiat exchange. There's always fees and market risks associated.

The USD historically was very stable compared to the rest. That in combination with the huge purchasing and military power the US represented while being reliable as a trading partner pushed the US dollar into focus.

The USA then spent a lot of time, energy, money and lifes (mostly others) in ensuring this dominance.

Think of it like a language: a Spanish and a German work together in English - not because it's impossible for them to learn each others language but because the overhead is too big.

Specifically to your example: no one wants to sell anything to Russia in rubles.

Imagine this scenario:

Hey, I'll pay you 100 YC (your currency).

Oh since we agreed on the price a few days ago it's only worth 80YC.

Oh now it's 120 YC but no one else will take the currency I'm paying you in except me.

Oh now it's worth 10 YC but we signed the contract, hurray!!