It's basically because these production lines are incredibly complex and getting tiny details right is important. When you have humans running a line, they can

It's basically because these production lines are incredibly complex and getting tiny details right is important. When you have humans running a line, they can notice and adapt to the countless possible issues that occur (like "the alignment here is often slightly out" or "tightening X here causes problems for the people in the next station"). Their top speed is theoretically less than a automated line, but they are quite adaptable and robust. Since automated lines are less adaptable and robust, so you have to get them just right. Any tiny little issue can cause surprising problems down the line and implementing fixes is quite slow because you can't just describe the problem and required fix to a human and leave them to work it out. I think the best way to set up a production line is to exclusively use humans at the beginning, and carefully introduce automation once the process is understood and stable