Becuase Skyrim's lighting and shaders are a joke. They look like their from a 2007 game or something. Very outdated.
Because there's extremely outdated texture depth methods, making all textures look flat and ugly.
Because the textures are extremely bad in general and a lot of the textures look like they could be from a 2002 game.
Because the game has poor view distance. Seeing grass and objects pop-up 50 meter in front of you is very bad for a 2011 game (sure you can increase the view distance of it, but it lags your computer more than it should).
Because shadows are probably the worst ones ever made for PC in a modern game?
Because a lot of the models are very low-poly.
Because some models are weirdly designed (mountains are not mountains, but rather lots of big rocks piled up)?
Skyrim can looks good in general, sometimes even really good, but because of what I said above, it can also look really bad in a lot of instances.
Technology-wise and graphics-wise, Skyrim is not impressive at all compared to what other games have nowadays. Since Oblivion, Bethesda's games have just looked worse and worse in standard to other games. This trend seems to continue.