barding Assuming GrapheneOS will run on phone hardware and remain a high-security system, somebody needs to build secure phone hardware and document how it works so that GrapheneOS can use the features.
Google has laid out a lot of money to build phones with strong verified boot, a strong key store, a flash controller supporting wrapped storage keys, memory tagging, etc. So far, other phone makers either have weaker hardware security or don't make it available to operating systems they don't own.
This problem isn't unsolvable in theory, but it would require at least one large company to do things differently, or a lot of money to fund some sort of open-hardware initiative. Such initiatives exist from time to time, but so far have not resulted in high-security phone hardware.
Also, assuming hardware were produced somehow, Google has agreements with repair shops in many countries, many cellular carriers, etc., that would not be easy or cheap to replicate.
At least in the short term, I suspect that if GrapheneOS on Google phones becomes untenable then people will end up paying a lot more money for phones that can run GrapheneOS, and repairs will be inconvenient.