I have often wondered about this myself.  

Wasn't satoshidice like a 1.6% edge or something?  I think off block chain dice games seem to have clustered around 1% but I keep expecting to see .88% or .5% but I don't.  I see the new PD3 is at .91% due to its jackpot feature.

Part of the problem may be that well heeled gamblers have shown the ability to win at dice sites and then walk away.  Just Dice had some early big winners that depressed profit below the expected level and the new dicebitco.in was in the hole after one month though it has recently soared to a profit.  So maybe the fear of variance is enough to keep the dice sites close to 1%.