Maybe Carl can weigh on how this works and why some dealers offer customer test drives and others don't. It could simply come down to insurance coverage and/or not wanting to deal with the potential for incident. Not all dealers are created equal.
For us, the car we're going to have here tomorrow is going to be our District Manager's car - the one he's being given to drive. So the insurance for that is his own insurance. When we went to the ride and drive, we had to show both our own drivers license's as well as our dealership insurance policy, in case we had an accident. (And that's normal policy when we're doing an event like that - been there, done that, many a time)
Now - if you are on a test drive here with me, my dealership policy is that I verify you have a valid license and insurance before we take off, because if you have an accident while we're on the test drive, it's on you as the driver (or whoever causes the accident), just as if you get a speeding ticket it's on you as the driver. And in nearly 6 years of doing test drives, none of the 3 accidents I've personally been in the car when they happened were my customers fault. (Two were caused by drivers fleeing the police - we have a wonderful neighborhood just north of here! The third was just a simple fender bender IN our parking lot. We had just pulled back into the parking lot after the test drive and were parking the car and someone backing out of a parking space backed into us.)