Gates doesn't know shit about how we make cars.
Each car is a massive investment, often requiring an entire factory to be built, a ridiculous amount of engineering goes into each one for cost control, safety, NVH, durability (both load and wear), CFD and so on, as well as virtually crash testing a car before they actually crash test the real thing.
The part about the 10th car costing the same as the 1000th is not necessarily true, as new NHTSA rules can be introduced at any time, which requires additional crash testing to existing vehicles, even if they have already passed.
These rules also apply retroactively to older vehicles as well, which, while they won't get crash tested, they can be recalled if NHTSA decides to create a new rule.
This essentially means that each car that is manufactured is essentially a financial risk to the company.