"Demonstrator" miles could lower the price a off-the-lot customer is willing to pay. Also, there are a LOT of people who can't afford a four-year lease on a Kia who get bored and go test driving expensive cars. "Lookie-loos" is what salesmen call them. In addition to putting wear and tear on new car inventory, the lookie-loos also keep the salesman tied up with them and missing "real" customers who come in to the showroom. If you don't have an ongoing relationship with the dealer's service department or a salesman, you're probably out of luck.
I test drove an E36 M3 in 1994 at an invitation-only event put on by BMW NA at an off-site, a Class-A office park. This was pre-internet, and I'd called BMW's 1-800 number for a brochure. I also subscribed to the Wall Street Journal and Car and driver, and I had perfect credit. I suspect being on all four lists is how I got on their invitation list.
The master of ceremonies was a BMW race car driver. During the punch-and-cookies session before the test drive, he went around the room asking what we were currently driving. When I said "Nissan Sentra," the room got quiet. But, then I said it was an SE-R, sort of the M3 of Sentra's. The race car driver said that he'd done some PR work for Nissan when the SE-R came out and that it was a cool little car. Then, everybody stopped looking at me with the "Who let him in here?" look on their faces. I ended up buying another SE-R.
I ordered an E46 M3 sight unseen: put down a deposit in July 2000, spec'ed the car in August 2001, took delivery in November 2001. I did get to drive one at a Drive For A Cure event in July 2001. But, they quickly took M3's out of the Drive For A Cure fleets due to "customer behavior problems." Imagine that. I made sure I was the second test drive of the day, before the local traffic cops figured out what was going on. The first test drive of the day was the Virginia Beach cop who moonlighted as the dealer's night watchman. I rode shotgun with him. I took him with me on my test drive as insurance, promising that we'd stop by Krispy Kreme on the way back... my treat.
If you can afford an M4, you can afford a trip to Spartanburg. The inverse of the converse of that is also probably true: If you can't afford a trip to Spartanburg, you probably can't afford an M3/4. That's a good reason for a dealer to not allow you to test drive one. Porsche has also started the factory-center-test-drive-for-a-fee programs at their LA and Atlanta centers. I'd bet you can't get your local Porsche dealership to let you take one out for spin, either.
The factory center test drive and "bespoke" production orders is becoming the norm for high performance cars. The factory-center test drive also means that your dealing with a car salesman is limited to just a few minutes for negotiating price. The car salesman as we know them might go the way of the travel agent.