It's a market size problem.
For consumer motherboard OEMs, only AMD effectively has ECC support (Intel's has been so spotty and haphazard from product to product), and of AMD users, only a small number care about ECC.
So motherboard companies, being resource and time-starved as they are, don't make it a priority to address such a small user-base.
If Intel started shipping ECC on everything, it would go a long way towards shifting the market.
Yes it is. The problem is they dont really advertise it. I'm not certain but it might even be standard on AMD chips, but if they dont say so and board makers are also unclear, who knows...