Because they do. A 5970, 6990, and 7990 all are crossfired internally and behave as if they are crossfired dual gpu. They do NOT act like a single gpu card. If your game only supports a single GPU, your dual GPU card will only see a load on one core, not both.
It's relatively same issue as if you ran sli. But the game stuff would be an issue on developer side.
I always used nvidia; since getting in btc tried out radeon for gaming. Let's see ..nvidia took over the market with cooler graphics card with stock fan and almost no drivers issues.
on the other hand amd's getting better..
anyways my point is....nvidia would have to get ready for gtx790...
or even more..sooner mobile computing power will overcome desktop power..