Because we send out the material with what is ready when it is needed. If we had waited to have final art before soliciting pre-orders, people would still be waiting and the announcement would not have been made at E3. Once the final art is finished, sites will be updated.
It is all a question of timing.
Have you not seen other games or toys, statues, comics, etc with "not final art" when solicited? I buy a copy of Previews magazine every month to help me plan what to buy for upcoming comics and toys and there is always materials liek that inside. It is what is done in the industry.