They do, however, do Battleforces every year, which are coming soon. Those contain models from one faction at a significant discount (usually 4 factions). There are also boxed games that do the same, such the Dark Imperium starter set, Burning of Prospero or Gangs of Commorragh. These may contain two opposing forces so you'll want to find someone to take the other half if that's the case.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.