Quoted By:
Nobody was aware of what Eternatus was besides Rose and the entire point was that hidden behind the legend was the actual cause of the Darkest Day, which was Eternatus.
Additionally, we don't know how the Regi legend applies to Galar at all yet so who the fuck knows. Maybe you stumbling on them is the first anyone has ever heard of them. Maybe they're a very localized legend in only a section of Galar while the Darkest Day and the events of the two heroes were well known.
