So a fashion designer can mass produce clothing? So an interior designer can build a house?
This designer should has never held.
For example designers and developers both use the computer as their primary medium of working. Their outputs resemble each other very closely, despite having a different underlying form.
Contrast that to the interior designer building a house, well those are different mediums. There is no efficiency gain from the interior designer designing the plan and also implementing it. Where as with a designer working in code there is one.
Fashion designers do indeed make clothing by hand, it's a very important part of their craft. This example disproves your stance.