Why is the term "open source" not applicable to any context in which the design of a product is distinct from its final form, and in which that design is conven

Why is the term "open source" not applicable to any context in which the design of a product is distinct from its final form, and in which that design is conventionally covered by copyrights or patents? If an architect released a set of standard building plans under an open license, would that not be legitimately "open source"?