In the case of passwords, if the attacker watches even one transaction, the key is instantly revealed because it is sent in plaintext. Thus the valid reason for cryptography is not valid for passwords. But now days every time I have to remember something new, it seems like I have to forget something else in order to make room. I also find it hard to forget my old passwords once I go to the trouble of remembering them. The last thing I want in my old age is to recall 50 years of new passwords changed every month because of some regulation.
Most people have to write down a new password in order to remember it.
The more often we change them, the larger portion of the time we have them written down somewhere. Maybe more importantly, I can get into all of them without having to remember hundreds of random numbers or write them all down. Whenever I change one of my passwords, I have to change scores of them. In one case, I have a network of 50 computers that I manage. If you make me change all of those passwords once a month, I'm not likely to want to manage the network for very long.