Because they are financed.
But the post FairTax final price of big ticket items is most likely to be the same, or nearly the same as pre-FairTax final price because of the levels of production and opportunity to embed tax cost. Once removed, the out the door price should be nearly the same.
Even if we assume, arguendo, that the out the door price is higher (which I don't), the cost to finance should be lower unter the FairTax as well. Please see: http://www.fairtax.org/pdfs/interestrates.pdf