Fairphone fails to properly sign their own operating system. They use the publicly available (!) AOSP test private keys instead of an actual secret key. This breaks fundamental security features of Android like Verified boot, rollback protection, etc. They're also pretty slow with updates, including important monthly Android Security Bulletin patches. This is just the bare minimum for any OEM, and Fairphone fails to properly implement it.

Google goes above and beyond, and offers cutting edge hardware security in their Pixel devices. They have features that currently can't be found in any other Android phone, like ARMv9 MTE (hardware memory tagging), the Titan M2 secure element, which supports Android StrongBox, the Weaver API and comes with insider attack resistance. GrapheneOS takes full advantage of these features, and combined with their numerous software security improvements offers the most secure mobile OS + hardware combination on the market. You can read more about all the hardening of GrapheneOS on their features overview page: https://grapheneos.org/features
Plus it's degoogled by default, so it doesn't come with any trackers or bloatware. You can opt to install Google Play services, but they are confined in the standard Android application sandbox, just like any other user-installable app. They don't get any elevated privileges like on other Android-based operating systems.