> why am I required to purchase a vehicle equipped with airbags?
The same reason that you can't sell food that's sweetened with lead. You can add lead to your own food if you want, just like you can drive a car without airbags.
Yes, keep lead out of food. The comparison is not accurate because the risk-reward profiles of lead-in-food and airbags-in-cars are misaligned. Food is consumed under all circumstances, whereas airbags are only employed during an emergency situation.
You're also right that I can drive a car without airbags, but the point is that you can't buy a new car without airbags.
The same reason that you can't sell food that's sweetened with lead. You can add lead to your own food if you want, just like you can drive a car without airbags.