This is a misunderstanding. It's just that Google Pixel are the only devices that have this level of hardware security engineering AND are open to thirdparty roms like grapheneos to a decent degree.
Samsung Flagships and Iphones seem to have similar level of security engineering in them (Pixels use Samsung CPUs essentially) but aren't open to the required degree for third party roms.
There's nothing else on the Market that delivers on that Level. The GrapheneOS guys are working with someone one a potential custom phone to get the required level of hardware security but nothing has materialized. Companies like Fairphone are free to deliver hardware that is competitive in the security space and i'm sure that the grapheneos team will consider them then. But until anyone else does i'll keep buying whatever phone grapheneos wants me to buy, i don't care.