I think companies that create copy protection systems would have a problem with loaning or renting games. They would have to come up with a time limited key, to prevent people from playing the game after returning the CDs. Then they would need to figure out which CDs were owned and which were rentals, either by distributing different versions, or by keeping track of how many CD keys are requested for a given CD. They would need to put a lot more work into key management for this, when they know they are likely going to get less money out of it through lost game sales. Long games or those with a good replay value could be sold based on people renting/borrowing the full game, but I don't think that covers the majority of games.
