There is a way to express one million as M̅ (horizontal bar above M for M * 1,000), but I think this was a later addon that was not used by the Romans. They probably rarely had to deal with numbers in the millions and if they did, they could just express them as a product like M * M
This is not the case in the United States.
k, M, B, T, Q seem like easier to read suffixes as opposed to having to count the number of “M”s.
You are correct the others are easier, clearer, etc. But banking goes back centuries, millennia even. When an entire industry uses a certain nomenclature, no individual participant wants to go through the hassle of trying to change everything.