Why? Pixel, like Nexus before it, are still provided as a development platform and baseline by Google as to what they think Android devices should be.

Unfortunately brands like Samsung that come close are hostile to devs/consumers by breaking their hardware through Knox and efuses when the bootloader is unlocked. Other OEMs cut even more by not offering any parity in regards to security focused hardware and reducing it from said baseline.

If other OEMs provided the same or better hardware and alternate OS support then supporting them wouldn't be a problem. However the question is usually misdirected as to why we don't support other devices. The real question is, "Why do they not support us?"