When it comes to the number of months in the year, 10 is an entirely arbitrary number because, unlike the metric system, there's no opportunity to leverage the convenient zero-shifting property of a base-10 numeral system (the concept of "kilomonths" and "megamonths" is incoherent as a unit of measure given that the number of days in a year is not divisible by 10). Might as well just smooth out the current 12 months (12 being a more highly composite number than 10) to 30 days each and then save the leftover days for a worldwide party outside of time.