Because if you knew the manufacturing that went into those cars; it's not practical nor is it cost effective. You partly buy a car for the comfy ride, not to be squished into a reinforced seat, strapped in with a ridiculous harness.
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A harness is hardly ridiculous, especially if it saves lives. I'm not suggesting each car be fitted with NASCAR equipment; I'm suggesting that a single strap across the chest and waist and a airbag are obviously not as effective as they need to be.
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Mainly because people don't use them in the first place. A harness brings up its own set of difficulties if you choose that route. Ever wonder why drivers wear a helmet and neck brace? With a harness, more of the energy in a crash is transmitted to your head and neck, which aren't restricted.