It's because in this temple of lean manufacturing, Tesla uses far more workers than NUMMI employed to build far fewer cars. In 1985, its first full year of production, NUMMI had 2,470 employees and produced 64,764 vehicles — about 26 vehicles per worker per year. By 1997, it had 4,844 workers and produced 357,809 vehicles — about 74 vehicles per worker per year.
Tesla, on the other hand, had between 6,000 and 10,000 workers in 2016 and manufactured 83,922 vehicles. That puts its vehicle-per-worker number between 8 and 14, about one-seventh the efficiency of NUMMI at its peak.
Tesla isn’t pushing the standards, they’re not even playing in the same league as Toyota and other established manufacturers. But they sure are good at hyping their flawed methodology.
What's surprising is that Tesla is also having trouble with battery pack manufacturing. That's far simpler than building a car. It's a good job for robots, building identical units made of a large number of the same components.
And I would not be surprised if part of this will be a massive simplification of the current "cable trees" with cables from each component to the central fuse board to something like 2 24V power rails and 2 high speed data buses (one for safety critical stuff, the rest for everything else), with plugs every 20-30cm or so, fuses inside the components themselves and everything controlled with intelligent boards in the components.
Installing the cabling tree is inarguably one of the most complex parts in building (or, for what it's worth, servicing/maintaining/extending) a car and also the part where you right now need humans in the loop. By including the power rails e.g. directly in the car's composite frame and only running a tiny strand of wire for the bus you save a boatload of money.
Musk is trying to beat the standard.