M means milli in Latin, as in thousand (kilo comes from Greek). Millennium, millipede (thousand legs), and so on. Million just means a thousand thousands. That is why in most financial institutions, M means thousand, and MM means millions, literally thousand thousands.
Which is weird to me, because MM in latin numerals is 2,000
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
Yes, for billions at least. Never personally saw a bank transaction for a trillion dollars.
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.
Or perhaps institution see some value in keeping a consistent denotation when you have records spanning hundreds of years, multiple counties, and multiple languages.
