With the plumber analogy the bill usually breaks out parts and labor and labor is, well, 100% labor.
With a more Japanese-like job like salaryman at a car manufacturer, the cost of labor per car is extremely low, like $1K to $2K per car regardless of size or cost of the end product, so if you have to bury $100 somewhere in a $60K pickup truck nobody is going to notice, so you end up with "do nothing rooms".
Also don't forget "do nothing rooms" are not Exactly a wasted expense, they are purchasing an option on future labor. If labor demand increases 10% next year for whatever reason, and you fire a guy with 10 years experience, you have committed a major management error, whereas having 10% of your workforce do nothing means you bought an option that fundamentally only costs $100 per automobile sold, so that's pretty cheap insurance in case you have to ramp up or someone dies or unexpectedly retires or something.
With a more Japanese-like job like salaryman at a car manufacturer, the cost of labor per car is extremely low, like $1K to $2K per car regardless of size or cost of the end product, so if you have to bury $100 somewhere in a $60K pickup truck nobody is going to notice, so you end up with "do nothing rooms".
Also don't forget "do nothing rooms" are not Exactly a wasted expense, they are purchasing an option on future labor. If labor demand increases 10% next year for whatever reason, and you fire a guy with 10 years experience, you have committed a major management error, whereas having 10% of your workforce do nothing means you bought an option that fundamentally only costs $100 per automobile sold, so that's pretty cheap insurance in case you have to ramp up or someone dies or unexpectedly retires or something.