> risk of cert compromise and forgery increases over time.

And what if the certificate is compromised before it expires? Right, there's a revocation mechanism for that. So why expire them then if they can be revoked anyway IF they get compromised?

The reason why domain TLS certificates expire is that domains can change owners. It makes sense that it should not be possible for someone to buy a domain for one year, get a non-expiring TLS certificate issued for it, and then have the ability to MitM its traffic if it ever gets bought by someone else later.

Domain certificates are sent as part of the connection handshake, so them expiring is unnoticeable for the end users. However, root certificates rely on the OS getting updates forever, which is unsustainable. Some systems lack the ability to install user-provided root CAs altogether, and some (Android) do allow it but treat them as second-class.