Because it's the most efficient way to put things together if you're doing many identical items with a lot of human labor.
After the development of mass production assembly lines, the next step is automated assembly lines. These lines use robotics instead of human workers to build items.
ships
Wasted motion is eliminated
Wasted motion is eliminated
Wasted motion is eliminated
Wasted motion is eliminated
Assembly lines
One of the first automobile assembly lines was started by Henry Ford after Eli Whitney had used this for the creation of guns, Henry saw fit to use this himself, the company was later on named Ford
Almost everything manufactured today is built on some type of assembly line. Most assembly lines use only machines and robots to build products, but there are some companies, who make higher-end quality products, that use human labor in the majority of the process.
Various parts are made on assembly lines but the engines are put in by hand.
technology
Assembly lines are one innovation that have increased efficiency in production.