Because other months went to 30 or 31 already, so there was room for it in calendars and people never assumed months are always the same length.
60 seconds in a minute, OTOH, is a very strong assumption in many systems and people's minds.
60 seconds in a minute, OTOH, is a very strong assumption in many systems and people's minds.
I really don't think this has much to do with it. It would predict no problems trying to add a November 31 to some year; I tend to make the opposite prediction.