There's no point in making dungeons labyrinthine because people will always just find the shortest and fastest route to the end. Even if you were to compare MMORPG dungeons to single player RPG dungeons, you have to keep in mind that in singe player RPGs the dungeons are the meat of the gameplay, whereas in MMORPGs they are just one piece of the bigger platter of content. MMORPGs also tend to focus more on the boss challenges, not exploration, since exploration is kind of useless in a game where you are meant to do the same place multiple times.