I had wondered about why the rabbits around here don't seem to have a rabbit warren, and the answer is that they are not that kind of rabbit as they are 'cottontails'. Wikipedia says:
Many rabbits dig burrows, but cottontails and hispid hares do not. The European rabbit constructs the most extensive burrow systems, called warrens. Nonburrowing rabbits make surface nests called forms, generally under dense protective cover.
I remember seeing an enormous rabbit warren on a hillside in the Yorkshire Dales. I was chaperoning a school field trip. One of the boys was very excited when he saw the warren and shouted, "Oooh, miss! I wish I'd brought me ferrets - we could'a had rabbit for dinner!"