What does reducing validity actually solve? The same private key is presumably used by all certificates and CSRs - if the private key is compromised then a renewed cert doesn’t solve anything.
The only thing I can think of is domain expiry. Someone could get a certificate, sell their domain, then continue using the cert until the certificate expires.
My tinfoil hat says it's about control. Forcing people to return for another blessing from the central authority, more often. The same reason church is once a week, and not once a year.
Am I missing something here?