There's a reason pixels are preferred, it's not some kind of malicious conspiracy. The most common sense reason being that there's a lot of overlap and cross-pollination of devs in the android world. between Google and graphene os in particular.
Pixels are also targeted because it's a mass-produced flagship with decent specs that is the closest thing to being already rooted off the shelf. It's the path of least resistance. Plus the used market is robust. A used carrier unlocked pixel 1 or 2 models behind the latest one can be obtained for several hundred dollars cheaper than it originally retailed for.
It takes effort to support additional brands/models.
Most brands lock their bootloaders and make "owning" the device difficult.