From India, Bangalore
From my perspective, the design engineer takes a concept (his/hers or others) and brings it to fruition by application of his mental abilities without any physical exertion.
The factory worker, on the other hand, has to take that concept and make a product using both his/her mental capacities as well as physical capabilities. Problems encountered in the production process require the workers to resolve them.
It is inefficient to have an engineer "stand by" to address potential problems.
From United States,
From India, Pune
From India, Agra