[Build-It-Break-it]

Because it won't have a VIN number for a 69 camaro (or what ever body you buy)etc. It'll be registered as a "specialty" vehicle I believe and you can't swap your VIN number from your rusty car to the new body either it's illegal.

When your done building the new body with "specialty" registration it'll be either hard to sell because it's not a real 69 or the value of the vehicle won't be there either.

That's why people restore their rusty Camaro. The value can only go up even tho it might seem to cost more to build up front.