Updating firmware from a system that is in an unknown state has high risks, especially if untrusted software is being run (like on most Linux systems). What if the user pulls the power or there is not enough juice left in the battery when an update has started? Many consumer devices don't have fallback firmware to account for such issues. Not being able to revert cleanly to a last good known firmware if something goes wrong will be an issue.
> Why do you believe firmware shouldn't be updated regularly just like we're used to do it with regular software?
OEMs quite frequently push updates that break something since they can't account for all of the configurations that their hardware will be used; sometimes they push botched uCode updates or other components they can't fully account for.