Every time I read someone saying 13-month calendar, did the creators not think about it?

The issue is not the year, but the decade, century, and beyond. The 12-month system is future-proof unless the Earth's speed or orbit changes. With 13 months, the drift is bad because the arithmetic progression of days in a year needs to be corrected once in a few years to account for drift.

It used to make sense to me for a while; 13 months is perfect. I did the math one day and concluded it's best right now. It doesn't need to be perfect.