Same reason car dealerships don't manufacture, and grocery stores don't farm and process food."...while adding little to no value to the deal."

Generally,  unless there are additional restrictions set locally, Mary  may move in to that commercially zoned area since her residential use is higher than set by zoning.Now, if Mary starts a trailer manufacturing operation, building utility trailers, she will most likely be out of compliance  with the commercial zoning, since manufacturing is "industrial" use which is the lowest use category (light and heavy industrial and those are broken down to specific operations as well).

The store at the mall doesn't typically buy directly from the producer/manufacturer.

Law enforcement suspected methamphetamine was being manufactured there although I later found out that it was in fact DMT. 

I'm sorry, but I can't understand your sentence regarding Cleveland.Cleveland, Youngstown, and the rest of the rust belt all declined for similar reasons--the loss of manufacturing jobs, which were the driving forces of the economies in this area.

Known as democracy of Arms in ww2 (educate yourself why they received that name) Number 1 for Manufacturing in USA Infrastructure of Detroit and even surrounding suburbs is built for manufacturing, trucking and import/export.