"So, very hard to do, and not enough of a business justification to do so (combined with old school management that don't really understand the value of open source). That is it."

I think this is your best argument. I believe one of the Big Two did open-source a large aspect of their FPGA's or software a while back. Maybe the bitstream. Nobody cared and they didn't make more money. So, they have no incentive to do that plus negative results that will probably happen.