> I wonder why nobody has set up, e.g., a good Yocto CI/CD base image pipeline for custom hardware devices

The near-infinite flexibility of OpenEmbedded/Yocto is a blessing and curse.

The Yocto autobuilder [1] covers board families, but hardware-specific layers [2] and BSPs are maintained by vendors and consultants [3].

Wind River Linux [4] offers long-term commercial support for regulated industries, derived from Yocto LTS, https://www.windriver.com/products/linux

Some work was done on a "Reference Binary Distro" to make Yocto more accessible to new developers, https://www.youtube.com/watch?v=7plHWCHGv1c

Funding has slowly diversified beyond hardware vendors, e.g. https://www.sovereign.tech/tech/yocto

[1] https://dashboard.yoctoproject.org [2] https://layers.openembedded.org [3] https://www.yoctoproject.org/community/consultants/