I think the primary distinction is that a weapon in a criminal context is typically something that is used to threaten/coerce someone, or to enable you to cause (more/more severe) physical harm/incapacitation in a physical altercation.

Date rape drugs aren’t used to threaten/coerce people, and whilst they can cause harm, it is generally not the intended goal when someone uses them. And intent/willingness to use a weapon to physically harm someone, in my opinion, is a relevant distinction to relatively “”“peacefully”“” knocking someone out. Of course committing date rape is still an utterly horrific thing, and people who do it should be charged and held accountable to the fullest extent of justice, but it is still different from threatening someone with a weapon and forcing yourself on them. (Also, whilst I have no actual data on this, it seems logical to me that a conscious victim is far more likely to receive (more serious) injuries as they struggle, vs. an unconscious one)

So whilst I agree that classifying date rape drugs as weapons is a good move, there definitely are relevant distinctions as to why drugs are typically not considered weapons.