There's still no incentive to do this outside of server hardware (Red Hat doesn't support rebuilds of RHEL). Why bother making a firmware stack that allows anything to boot when you can just modify the Linux kernel to work on your non-standard hardware and work around any bugs, publish an Ubuntu image that you'll only update a few times and then call it a day. Then, maybe, just maybe, support for your SBC will be upstreamed a few years later, but long after it's actually useful.
Look how messed up and fragmented Android is still, even Windows Phone had a UEFI stack that in theory could allow all Windows phones of a certain architecture to have a shared image (although I don't believe this was fully achieved).
Imagine if Windows or Fedora for x64 had to be repacked for every single motherboard/CPU combination, it'd be insane, yet somehow it's fine for ARM, what a joke it's been allowed to become. I think electronic waste regulation is going to be needed to sort it out.