compounded by, in D&D's case, the fact that most of their core material uses completely generic terms; fighter, cleric, strength, wisdom.

Excepting a few, most of their monsters are generic fantasy monsters too or random combinations of "let's stick this head on this body".

The stuff that is copyrightable they do a terrible job at monetizing (Forgotten Realms, Greyhawk, Eberon, various planes, higher powers, etc.)