They were not written to use DX11. Most games are written in DX9, because almost everyone has a machine capable of DX9. If they write the game in DX11, they usually have to make a DX9 version too, which costs a lot of time and money to do. Only the most cutting edge games will typically offer DX11 as a game engine option.
Metro 2033, Crysis 2 with the patch, Alians versus Predators, Dirt 2 & 3 and Battlefield 3 are a few examples of games with DX11. Most games will not use DX11.