They aren't normal computers in the same way x86 is. They usually need proprietary drivers in custom boot processes, forked kernels with patches, etc. I'm yet to see a single SBC I can just download the iso from the debian website and get going. You always need some custom build.

ARM boards need a custom kernel and bootloader. These aren't things managed by the distro. The userspace IS managed by the distro and IS standard. Using yocto to manage userspace maybe made sense 15 years ago, but it has long since become far more trouble than its worth. Debian supports most architectures out of the box, has good cross-building infrastructure, many thousands of ready-to-use packages, and is non-weird.

Debian handles userspace fine but there is a lot of image customization needed a lot of the time. I have experience with developing images for a beaglebone black, where I forked the bespoke image builder (to create user accounts, mount points, set up udev rules, device tree overlays, ssh keys, etc) and that was a pain to maintain. I suppose I could have create debian packages for some of those things, but I would still have needed to set up repositories. Now I'm using an SoM with Yocto and while the learning curve is quite a bit steeper, it is much easier to customize for our experiment's needs. Also, AI assistants are really good at spitting out yocto recipes (that usually don't work immediately, but gets you 90% of the way there).

You're missing the point that when you're building the product, and not using SBC just for fun, the image you are building, and yes even the userspace, is highly customized. Running a full-blown Linux distro is a non-requirement, and often incorrect.

For more mainstream SBCs, totally agree. However a lot of the niche industry boards (especially new ones) only support Yocto out of the gate. Debian might be "supported" by the SoM/CPU vendor but manually reapplying kernel patches and praying it boots is a PITA.

I agree. In the past, I have successfully used Debian and Alpine for embedded. Never needed to compile OS kernels or standard DLLs, other people already did and published in these package repositories.

Except you may have just a 512 M industrial sd card to run on. Or even less.