Because there are too many standards[0]! If only someone would invent one more standard that everyone could follow...

The Wikipedia on long and short scales[1][2] for the root of the problem.

[0] https://xkcd.com/927/

[1] https://en.wikipedia.org/wiki/Long_and_short_scales

[2] https://grammarhow.com/m-vs-mm-million/