Code libraries and your house do not fall in the category of "things that give you game assets without you having to make them" because they do not give you game assets without you having to make them. (Code is not considered a game asset.)

Asset packages are in the category of "things that give you game assets without you having to make them", because they give you game assets without you having to make them. The assets in the package are, by definition, stock assets.

It’s a metaphor.