I think it comes from a basic lack of respect or understanding of the actual skill requires to work an assembly line properly. Highly educated creative class are trained their whole life to look down with disgust on the people who do repetitive labor. The name shows it.. They call it unskilled. And yet how many of them could do it?
It takes an enormous amount of focus and concentration, the ability to learn complex routines quickly, to create efficiencies without breaking rythyms or rules. All while being payed less and treated like a second class employee.
Why is toyota different? Because they dont think like this. Line workers are thought of highly. Their ideas are integrated into the system. Most of all they are not disposable cogs