"Its one thing to render a large environement, its another to make it completely immersive and still provide good FPS etc when playing online on a full server. None of the engines you mentioned (Crytek, HF2 or Doom3) have proven their ability to do that. "
Doom 3 was designed for indoors and seems like it has to be retro-fitted to do something like this; hence the MegaTexture technology announcement. It's simply because it was falling behind and licensees probably needed something more.
HL2 seems to be a little in the middle, but Far Cry failed purely on net code. I don't think it had much to do with large environments.
Hearsay disclaimer on the above statements.