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Yes if your device is dead and you have the facilities, take the disk out and wipe it on another machine. But not everyone has the know how, or the capability to do so.
That is only an option for some.
With solid state drives - such as tablets that is not going to be possible for many, including corporations with IT departments. And if the phone has died it will be beyond many. No actual disk device or memory to access anyway as it is probably soldered to the motherboard. And if you get to that level your warranty has been voided.
In some cases it may be best to physically destroy the device and buy another. It is probably time for a new one anyway - isn't that last years model?
As for buying new devices and finding that they are pre-loved, I would be complaining strongly. I actually think that is illegal in Australia, you need to inform people if the item is second hand or used even as a demonstrator.