When we start counting large quantities of $10's$, the number system varies by country/region:
- Europe/US: $10^3$ (thousand, million, billion are all multiples of $10^3$)
- Japan/China/Korea: $10^4$ (万, 億, 兆 are all multiples of $10^4$)
- India: $10^5$ (lakh), $10^8$ (crore = $100$ lakh)
In the 1600s, Japan used different groupings, and Europe used/uses the long scale. I am sure there are others I am not aware of.
As someone who has worked with different number systems, it is incredibly difficult to process large numbers if you are used to counting by $10^3$, and have to read written numbers that are based on $10^4$ (while not a perfect analogy, it requires conscious thought like switching from base $10$ to base $8$ or the like).
If Wikipedia is to be believed mathematical concepts for numerals were shared across regions. We all use base $10$, for instance. Given that these systems do change with time within a single country, why hasn't there been any unification between countries to formalize how many decimal places to group larger numbers by?
For what's worth, even the same language, like English, had two counting systems, since the British, just like the rest of Europeans, used the long scale $($million, milliard, billion, billiard, trillion, trilliard, quadrillion, quadrilliard, quintillion, quintilliard, etc$)$, whereas the Americans use the short scale $($million, billion, trillion, quadrillion, quintillion, etc$)$. Secondly, the use of myriads $($ tens of thousands$)$ is customary in Greek. Not to mention that dozens and gross were used up until not too long ago in human history, since they divide so nicely into the customary fractions: halves, thirds, and quarters.