Cars aren't cheap

Compared to the cost of designing a car then manufacturing hundreds of thousands or millions of them, a handful of cars are pretty cheap.

Depends on how far down you want to go with all the people sizes in all the various combinations of positions. I imagine the cost-benefit is based on the marginal change of information you get by changing the dummy size/shape, and based on their revealed behavior, they don't think more sizes add more information.

When I worked at Toyota, they used "number of Highlanders" equivalence when shaming people for not picking up their large format paper schematic prints.