An assembly line worker just puts parts together - they don't understand why those particular parts have to go together in that particular way to achieve the desired result. Engineer is the person who does.

And engineers have always been assembling boxes together. At first those were the most basic components, like levers. Then we figured out about standardized parts and interfaces, and components got a lot less basic. But, fundamentally, what's the difference between an engineer fitting two cogs together, and an engineer fitting two pieces of electronics?