Comment by microtonal

I don't think they can, even if they wanted to. From the GrapheneOS device requirements:

Device support code updated to new monthly, quarterly and yearly releases of AOSP within several months to provide new security improvements (Pixels receive these in the month they're released)

Fairphone's hardware and software is developed by a Chinese ODM (T2Mobile), who even have difficulty pushing out monthly patches very timely. They don't even do QPR2s, major Android updates are very late (usually almost a year), they rarely do driver firmware or kernel updates. There is no way they could fulfill this guarantee unless they started doing software development in-house and paid Qualcomm for monthly firmware updates.