Cars already purchased: Their business would collapse overnight. That's called 'Bait and Switch' - not a good business model.
New cars: Sticker shock would put off prospective buyers. It's a lot easier to get people to pay $73 grand up front, rather than $70 grand plus $1200 for heating, then $800 for air-conditioning, plus $500 for GPS, plus $700 for entertainment, plus ..... well you get the idea.