At least they have a good reason for it. GrapheneOS exists mainly for security, and if the firmware is out of date that compromises that goal.

Why are older devices no longer supported?

GrapheneOS aims to provide reasonably private and secure devices. It cannot do that once device support code like firmware, kernel and vendor code is no longer actively maintained. Even if the community was prepared to take over maintenance of the open source code and to replace the rest, firmware would present a major issue, and the community has never been active or interested enough in device support to consider attempting this. Unlike many other platforms, GrapheneOS has a much higher minimum standard than simply having devices fully functional, as they also need to provide the expected level of security. It would start to become realistic to provide substantially longer device support once GrapheneOS controls the hardware and firmware via custom hardware manufactured for it. Until then, the lifetime of devices will remain based on manufacturer support. It's also important to keep in mind that phone vendors claiming to provide longer support often aren't actually doing it and some never even ship firmware updates when the hardware is still supported by the vendors...