Why though? There was a recent post around here that I think really hit this home- “whatabboutism”. We don’t need Graphene to fulfill every scenario. We need it to do it’s job well, which is does, and if it doesn’t fulfill your needs, then pick something else like LineageOS which has much more broad support at the expense of a less secure OS.
You sure can compile Graphene on other devices with varying results, the supported/tested device list used to have things other than Google devices on it- part of this comes down to support and dev time too.