Can you give me a good reason why my almost 30 year old password is no good anymore? The only one I can think of is someone gets ahold of discarded unix hard drives that had weak hashes back then. Even if they get the password it won’t do much good in a rainbow table nowadays since almost every hash is salted. I would argue that with the maturity of such techniques the need for changing passwords is basically gone.
Changing passwords regularly is a tactic for mitigating potential damage if the credentials are discovered (there are multiple ways of doing this), or shared. Changing the passwords forces a potential intruder to have to be continually probing systems trying to discover the new passwords, and risk potentially being discovered themselves in the process.
Also, when the password changes attempts to log on using the old password will be recorded by the security systems, along with the source of the attempted logon. Those failed logon attempts are evidence of a possible intrusion, and security software will monitor the logs and analyze and alert on those events for investigation.
Having access to a set of credentials with a password that never expires allows an intruder to quietly access and monitor a system for months or even years without setting of those alerts or hitting the tripwires.