Why does GrapheneOS run only on Google Pixel phones? Super suss if you ask me.

Because Google was founded as a DARPA/CIA operation, and Google is all about spying, control and censorship (google jigsaw). I am HIGHLY suspicious that GrapheneOS would only support Google phones. This maybe made a bit of sense initially if the Google phones had certain new security hardware features that other phones didn't have, and fully documented specs, but at this point that excuse is getting ridiculous. I suspect there is some hardware "watcher" in those things, low enough that you'll never detect or disable it, and they see everything though it. Huawei phones probably have a similar "watcher" that reports back to China.