Genstabler, in simple terms, there isn't a complete set of paperwork to say that the design, build and continued provision of the aircraft is safe. It may be safe, it may not, but without the paperwork it is automatically assumed to be unairworthy.
Not wanting to marginalise the argument, it's like buying organic food at the supermarket without the shop being able to confirm, with complete certainty, that the food is actually organic.