computer simulation v. actual play

[bilyin]

In Omaha, according to simulation, preflop hand values are close together (much closer than hold'em hands preflop). So it seems you can play looser preflop because you are usually not much of a dog. But my experience is just the opposit. In Omaha it is vitally important to play good starting hands. Interestingly enough, in hold'em, a game with big differences in preflop hand values, loose play hurts much less.
Why?