Because changing calendars is expensive, difficult, and ultimately needs a very good reason to justify it. "It would be slightly more rational" is not typically on the list of why people have adopted different standards of any sort.

Historically, calendar changes occurred only when there were obvious practical problems with the previous calendars. So the Gregorian calendar (the last major calendar change in the West) was adopted when it became clear that the Julian calendar had enough "drift" to it that it did not track with the seasons over the course of centuries.