SecureBoot uses an existing certificate implementation which supported expiration, for a scenario where a having a reliable clock in unfeasible.

SecureBoot would have been better off with certificates that never expire. That's not a problem in cases where users (or organisations) manage their own hosts, since they can just changed the certificate when the previous one is no longer valid or leaked or whatever.

In practice, SecureBoot rolled out with a single CA for everyone, one controlled by Microsoft. This provides little value for anyone—restricting your computer to "only boot stuff signed by a third party" doesn't really protect from attackers in any way. They'll just boot into one of the many programs signed by MS. But because a single CA is used globally, you want expiration so as to roll them over every few years. But remember: there's no way to have a reliable clock. And so, we have the mess that we have.

The grand majority of Linux users could disable SecureBoot tomorrow and their system's security would not change in any meaningful way.