> I would rephrase it as why attesting that we have an unknown and outdated OS is valuable to the phone owner?
I am not sure if you're genuinely not understanding what the secure boot does, or if you're just venting about the situation with mobile phones.
The secure boot is there to attest that the OS running on your phone is coming from the manufacturer and has not been tampered with by a malware. If you don't trust the manufacturer or if the manufacturer doesn't update the OS frequently enough, then I guess you should look for another manufacturer. GrapheneOS is pretty much up-to-date.