It boils down to extremely strict security requirements only Pixels provide currently:
  • Relockable bootloader with ability to specify custom signing keys
  • Hardware security module (Titan M/M2) that can be used by other operating systems. GrapheneOS relies heavily on hardware security.
  • Long term support of 5 years with consistent updates
  • Each hardware must have supported up to date firmware.
They focus heavily on security first and their developers are completely against making compromises. They do want to work with OEMs to make their own phone someday though