There's something that you may have not considered...

Do you know why Linux and Windows boot on all x86 hardware pretty much without issues? Or why you can still boot Windows XP on modern machines (assuming you've drivers)? This happens because those machine have a BIOS/UEFI that work as an abstraction layer between the low level hardware and the system kernel.

On the ARM ecosystem every single new SBC requires someone to fork the kernel and implement device specific stuff into it, this makes the kernel increasingly complicated (bugs), takes time and resources. For the end user this means that when an SBC is released we have wait until someone does that work or use a questionable image provided by the manufacturer. Note that manufacturer provided kernels aren't just questionable, they usually don't get updates and you may be stuck with an old kernel for ever.

If we manage to get ARM vendors to implement a UEFI then any Linux kernel would boot and probably work just fine without any extra tweaks as long as the CPU is already supported.

This is not about what we can or can't do with well tested SBCs, this is about making sure new boards work out of the box without extra work, long term support is easier and we also get mainline support for the majority of Linux distros instead of just a few.