the car analogy is not suitable for digital devices
It is trivial to "wipe" an digital storage device in that it will appear 'as new'. There is no chance 'some bits to be left behind' as in your car analogy, which is a physical piece by piece process.
It is not only trivial to wipe clean digital devices, but also to install fresh and up to date software etc. The device likely left factory months ago, was sit ton on shelves months and by the time it enters the refurbisher's place it's software might be way outdated.
About the only reason this is not happening is that the refurbisher is not competent enough to do it, or just plain not performing their business duties.
Making it possible to wipe data from possibly dead devices means that parts of the device (storage) must never die. While that might be somewhat possible, it will make the devices much more expensive, and therefore will never happen.
About the only proper way to do this is to use some form of encryption for the storage. Once you don't known your password the storage is inaccessible and can only be wiped. Simple and cheap.
