(Who are “they”, by the way? It would worthwhile to give this some thought, as it is otherwise a bit crude to be lumping the entire population of 1 b plus into a single category…)

Anyway, my guess is that the “made in” label simply had to be on every product for US import requirements, so “they” put it on.

Then much later on, quite recently, “they” realized that a deeper meaning could be found in the concepts of “made in” versus “made by”.

So I would say it was a recent opportunistic discovery linked to a widely-used language pattern.