You're confusing real world lighting terms compared to 3D graphic terms. In which case it isn't ubiquitous. Note: I'm going to try and simplify it so I will leave out some details.
In 3D graphics, hard shadows are direct shadow cast. Soft shadows also include indirect lighting on the shadow cast. Technically this is only true with ray trace based rendering(like cycles path tracing and The new ray trace in Eevee)some render engines like older Eevee and game engines use work arounds. Term like light and dark are more on the artistic side of 3D graphics.
Side note: in some 3D software light and dark shadows are terms in reference to transparent/translucent mathematics used by a render engine.