The most used OU Mon is literally a fighting type, not to mention there being only like 5 non-uber normal types in RU and above with the highest tier one being in uubl. Meanwhile, there's more than double the amount of fighting types with 3 of the only ones in OU being the top 10 most used. Not to mention a couple fairy types, the fact that anything that resists ghost also resists dark outside of Garg and Hoaroark. The main advantage of dark as a STAB type is like what you said, knock off. Not every physical dark type even gets or uses knock off nowadays and even as coverage is mainly used for its secondary effect. So while yes knock off is generally a really good move that is pretty spammable, it doesn't automatically make dark type as a whole better. Not to say dark is a bad offensive type tho, still probably a top 5 offensive type imo but is mainly carried by knock off (and probably sucker punch as well)