I get why they do this, because downgrade attacks are a thing that are used to exploit devices remotely, but there are other ways to implement this, like what GrapheneOS does. Downgrading can also just be restricted to unlocked bootloaders as well via a software revocation list that gets deleted/bypassed upon unlocking.

There is no good reason for devices to use efuses to block downgrades unless they are trying to restrict user freedom a la consoles.