The main difference is that the license you get when you buy a CD is transferable, that is you can sell it to someone else when you are done with it, while Stea

The main difference is that the license you get when you buy a CD is transferable, that is you can sell it to someone else when you are done with it, while Steam explicitly disallows this. Transferable licenses create a second hand market which keeps prices in check, which of course publishers don't like at all.