here is why: you can only install it on guugl phones. why would guugl, the criminal data kraken, allow to deguugle a phone, their own phone which only they control?
guugl uses a proprietary chip called tensor. no one knows what backdoors and tracking tools they put into the tensor to track and trace you and sell your data to criminals like those who call themselves "the gobernment".
allegedly only guugle phones have those 'security features' which makes it impossible to install graphene on any other phones.
are all other phones insecure? i doubt it. nonsense!
you will argue that the new - not yet available - motorola phone will also allow the installation of graphene. motorola is owned by a CHINESE company - lenovo -which was sued in the usa because it had put spyware on computers of their customers. they are the chinese communist gobernment or they are at least very close to them, otherwise they could not operate in the way they do.
if something smells very bad, it is most likely bad.
therefore i do not recommend graphene. install e/os or even open source android AOSP like lineage on easy to unlock phones. if e/os is not available on your device, you will have to tinker with it more than with e/os if you need banking apps to work but it is worth it because you will not get tracked and traced if you set it up in the right way.