They didn't want you to rent it multiple times. They wanted you to rent it once, be unable to beat it, but be intrigued enough that you purchased the game from a store. If you could play and beat a game in a single rental, there was little incentive to buy it (so the developers thought, and I imagine had some data to back it up).

More rentals = more demand = more copies purchased by rental stores (I can't rent you the game you want if someone else has it right now).