I would say it's because an unrented car is a smaller drain on capital than is an unrented hotel room or an empty airplane seat. This is why agencies rarely ever run out of cars, unlike hotels and airlines. Yes, they might run out of a specific type (that was Seinfeld's issue), but that's not unusual, just like hotels run out of 4th floor two queen rooms and airlines run out of starboard aisle seats.

Rental agencies typically replace their cars after a certain number of miles, or if it breaks or is crashed. If a car isn't driven because of a no-show, the odometer won't change, and there's no chance of it crashing.

Only once was I glad that I made a reservation (aside from after-hours pickups) -- at OXR (a small airport), Hertz was sold out. If you didn't have a reservation, you would have to rent from some no-name agency next door with Dae Woo cars.

Other than that, making a reservation was merely a convenience for me. One time I did no-show because my trip was cancelled, and I was mad at Hertz, so I didn't call.