Well, floorplan financing is a finance vehicle like any other, so terms are different among participating providers. Also, I'm a bit unsure of what you mean by "sunk costs".
Each vehicle ordered into inventory is an advance on the line of credit that has repayment fees and interest. Each vehicle sold then translates into paying the floorplan provider the advance amount + fees.
In your example, I'm unsure of how you fail to see that Car B is already costing the dealership more money. But let's work it out anyway.
Car A just arrived. Car B has been sitting for 100 days.
Car A cost $20,000 and is on the lot for 0 days @ $10/day.
Total repayment? $20,000.
Car B cost $20,000 and is on the lot for 110 days @ $10/day.
Total repayment? $21,100.
If you were the dealer, would you want to lose that additional $1,100? I'm willing to bet it would matter to you.
Complex case:
You're in a floorplan agreement that requires repayment of advances in full (including accrued interest and fees) in n days. You damn well better sell enough specific inventory to cover this without losing profit. The longer specific inventory stays on the lot when you're repaying each period, the more screwed up your numbers become.
It is about both moving inventory and moving specific inventory when that inventory is costing the dealer as much money as 10 other cars that sit on the lot for 20 days each.
Moreover, the typical expected time-on-lot tends to be between 30-90 days. Floorplan terms are usually negotiated for these typical cycles. If a dealer is in a floorplan setup that is built for a 90-day max turnaround, and then has vehicles that sit around for 200+ days, that can add up to significantly higher fees per old car that eats into the profit margin. This is especially important with dealers who are selling based on a profit-per-vehicle basis, as opposed to the profit-per-level basis in use by volume dealers. Last thing you want to do (as a dealer) is strike a deal on a 200-day-old car that you had to strip to invoice price or below to sell, and then repay a floorplan advance + fees that exceed the sales price. It happens, but you still don't want it to happen.
More important still, if a dealer is unable to keep moving inventory off their lot (whether through consumer sales, fleet sales, or dealer trades), then they have more inventory eating up their floorplan, diminishing the amount of inventory they can continue to purchase to replenish supply.
Now, to balance all this out and try to create new profit centers that both put more cash in the bank and provide another opportunity to make money off a potentially negative car sale, we have the Dealer Finance Officer. An absolutely disgusting professional. But that is a different kind of discussion. You may already be familiar with how finance officers help increase dealer profits-per-sale.