Soldato
I think it's more that both companies have abandoned Crossfire and SLI - although it does seem AMD is working towards multi-core rendering from another angle, I'm not sure if nVidia are working on anything in that direction.
Actually they do. Started hiring people earlier this year, but any product to come from this is 5 years away from now according to NV.
Similarly to Intel. Their core replacement to something like AMD Zen, is 4-5 years away also.