First, dealerships don't really make that much of a profit on new car sales. At least not Honda dealers. Used cars averaged around $1000 profit, but new cars were around $200. Yes, they really do lose money on some sales. That's not to say that there weren't expenses that were being covered in the price that are exclusive to the dealership, such as the salesperson's commission, but the total profit going to the dealership per new car was low. One of the older, wiser managers, who had come from Toyota told me, "A properly-run dealership pays for everything on Parts & Service. Car sales is just the profit." There are a lot of lean years in car sales. I can attest that no one in management was panicking when gas prices hit $4/gallon and we couldn't give a car away (car managers don't hold back their emotions). Sales stopped, but people kept getting expensive oil changes.
Second, at least within Honda's North American division, you cannot just buy inventory. A dealership inventory is controlled in such a way that area dealerships are forced to compete heavily with each other. In order to sell a car, you have to have a car on your lot. In order to have a car on your lot, you have to order it from Honda. The number of cars you can order from Honda is restricted to a percentage of the number of cars you sold last year, something around 106-110%. You can't just take a big loan and buy yourself volume. To a dealer, inventory is life and must be protected. As a salesperson, I would lose so many deals to other Honda dealers who would lie through their teeth over the phone to sell. Dealers will cut their own throats to steal a sale from another dealer because each one is a net +2 in the inventory war. In theory, this is great for customers on cost, but terrible for customers on experience.
The owner of the first dealer I worked for had just sold the dealership to a Fortune 500 company. He spent years and years basically giving cars away until he had the largest inventory on the entire East Coast (a big deal for Honda) and then had something very valuable, something that couldn't be bought. Somewhat like how Amazon, in theory, operates.