Part of is that the manufacturer buys outside produced parts and those parts may be 60& labor and that is incorporated in the materials category.
I used to do complex estimating on very large construction projects and as most of the buildings involved are 60% (average) subcontractor costs our labor totals never carried the labor of first or second tier specialty subcontractors or material supplies as a “Labor” item in the cost estimate. Only our direct labor and all the indirect costs were carried. Often the indirect costs of labor equal or exceed the paid labor amounts.
Again you pulled that out of your butt. Parts are stamped out and very little human contact needed.
The engine block is the largest most complex car part. Here is video of one being made. It is 99% automated. Watch it!