This is one of the reasons they’re reducing the validity - to try and convince people to automate the renewal process. That and there’s issues with the current revocation process (for incorrectly issued certificates, or certificates where the private key was leaked or stored insecurely), and the most effective way to reduce the risk is to reduce how long any one certificate can be valid for.

From digicert.com/…/tls-certificate-lifetimes-will-off…:

In the ballot, Apple makes many arguments in favor of the moves, one of which is most worth calling out. They state that the CA/B Forum has been telling the world for years, by steadily shortening maximum lifetimes, that automation is essentially mandatory for effective certificate lifecycle management.

The ballot argues that shorter lifetimes are necessary for many reasons, the most prominent being this: The information in certificates is becoming steadily less trustworthy over time, a problem that can only be mitigated by frequently revalidating the information.

The ballot also argues that the revocation system using CRLs and OCSP is unreliable. Indeed, browsers often ignore these features. The ballot has a long section on the failings of the certificate revocation system. Shorter lifetimes mitigate the effects of using potentially revoked certificates. In 2023, CA/B Forum took this philosophy to another level by approving short-lived certificates, which expire within 7 days, and which do not require CRL or OCSP support.