That would make them considerably more dangerous for the driver. A roll bar will crack your skull if your head hits it and a 5 point harness will snap your neck

That would make them considerably more dangerous for the driver. A roll bar will crack your skull if your head hits it and a 5 point harness will snap your neck. Race cars have them because drivers are also wearing HANS devices and helmets to prevent those injuries. They are two very different approaches to safety that aren't really compatible.