You are assuming that there is a fast internet connection available at all times. If I put a disk into the console, I can play it immediately. If I start a download of a 2-layer DVD's content, 9.4 GB, then that is 10 hours to download it at a 2 Mbps connection. Even on a 25 Mbps connection, that is 50 minutes. These times aren't prohibitive if you have just bought a game and are downloading it overnight, but they are not sufficient to have it immediately ready.
Or, consider a LAN party. 5 people each bring their consoles, and 3 games per person. Using disks, you put whatever disk into whatever console, and start playing. Using download-only games, you need to wait until the correct console is available to use.