You called it a problem coming from the manufacturers. That implies they actively thwart it (woa, another new word! It's crazy how language works). But they don't actually have a problem with it; some of them are actively publishing the info needed for alternative OSes to work on their hardware and GrapheneOS needs only take it if they want to, but they don't. Who has a problem with whom here?

Again, not saying GrapheneOS is doing something wrong. Nobody's under an obligation. Just that, if someone wants to argue that one party is making a problem out of the situation, I don't find it fair to assign that label to every manufacturer on the planet besides google