(they may call it something different in each state, but that is the Manufactured Housing Community Association here in AZ).

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.

Not much - they didn't manufacture the product, they didn't transport the product necessarily, etc.