Samsung doesn't have the security hardware or ability to load a custom os.
Discussion
Samsungs hardware security offerings ARE the closest to Pixels and would be ideal candidates for support. HOWEVER their use of Knox/eFuses disables this HW and other functionality when the bootloader is unlocked in a hostile approach to alternate OSes making them unworkable as GrapheneOS devices.
Alas the result is a case of them not supporting us not the other way round.