Well, there are many arguments besides contacts with Law Enforcem Agencies.
One big argument is privacy. GrapheneOS is a very minimalistic OS without all of the bloatware and spyware that many manufacturer bundle with their phone. A freshly installed GrapheneOS is talking to only a very few numbers of server on the Internet, such as the software that provides updates for the OS.
Next argument is, reducing the attack surface. Having a very minimalistic OS helps here. Furthermore, GrapheneOS contains several hardenings, which makes it harder to sucessfully apply an exploit.
Does it make the OS bullet proof? No. When a threat actor with exreme amount of resources attacks you (such as the NSA) it is likely that they will find a way over time. But a reduced attack surface + hardenings helps to make it much harder for cyber criminals to compromize your phone. And when its to hard to compromize a system, efforts are higher than the possible profit.