Pixels are the only devices providing the required updates and security features. These requirements are listed here:

https://grapheneos.org/faq#future-devices

GrapheneOS is partnered with a major Android OEM working on improving their future devices to meet our requirements. The first devices with official GrapheneOS support from them are planned for 2027. It takes time and resources to make reasonably secure devices. Future generations can improve further including adding hardware-based privacy/security protections unavailable on Pixels.