Because you get exactly 5 weeks inside a month. We could use six 5-day weeks instead, but I think that having an even number of days in a week is convenient in

Because you get exactly 5 weeks inside a month. We could use six 5-day weeks instead, but I think that having an even number of days in a week is convenient in many cases (gym routines is one such example). Also, with 6-day weeks, the quasi-month will not be longer than a week, so "holiday month's Monday/Sunday" will be unambiguous.