I don't know about SafetyNet specifically, but it could be that it has to authenticate itself to the apps that call its APIs. E.g., I guess it could be code-si

I don't know about SafetyNet specifically, but it could be that it has to authenticate itself to the apps that call its APIs. E.g., I guess it could be code-signed and that the apps that use it have to check the signature. That uses a private key that "FakeSafetyNet" couldn't duplicate (well, assuming the SafetyNet people could keep it a secrect). It also incorporates a hash of the binary being signed so that FakeSafetyNet couldn't get away with just swiping the signature from SafetyNet to present as its own (the binary hash of FakeSafetyNet wouldn't match the signature). I'm just speculating here, though.