Safety, need to still meet current minimum standards when rebuilding a vehicle. If someone rebuilds an old vehicle they require a mechanics signature saying the vehicle is safe for the road. If the vehicle ever gets into an accident and even after the mechanic ok the vehicle is on the owners back to prove their vehicle did not contribute to their injuries, or that of others, if it does not meet current minimum vehicle standards. So if you think having an antique vehicle is worth a lawsuit if you ever get into an accident that will leave you broke, well there is a good reason why they have a modern chassis.