Hard to guess really.
Maybe slope-scaled depthbias(correct me if I’m wrong, but UE4 does not use slope scaled depth bias for shadow depth rendering) brings in more issues, than it solves, or perhaps due to cross platform complications.
Or maybe it is simply considered that the issue is so insignificant in the final picture, that it does not deserve any attention.
Latter however is not true, because the problem is clearly noticeable on most curved surfaces in environments with a pronounced dominant light.
As to some advanced techniques, such as using DDX/DDY for more precise bias calculation, they might be not worth it performance wise.
Normal offset can be done on per-material basis.
In any case, I think that the problem deserves some attention.