Just like with everything else F/OSS, the issue is always either funding, availability of skill, time conflicts, or lack of interest. Plus with a lot of these devices, it’s not super simple. More often than not, while you do get the kernel source, there’s still proprietary shit that the device won’t work without

Newer Pixels use Google’s custom Tensor chips which have very limited documentation and closed-source firmware blobs. The verification boot process is also more locked down with each generation, making it harder to replace the OS without breaking security features. Mainlining these devices is a massive undertaking without proper documentation from Google.