NVIDIA GTX 595 (picture+Details)

[zalbard]

Whit this logic, you are dismissing the idea of double-GPU too.

No, I am not.
There is a reasonable limit of GPU sizes. Staying within that limit gives decent yields, thus allowing to keep price reasonable.
If the chip is too big and yields are bad, then making such chip is becoming too expensive, and it makes more sense to make a card with 2 smaller and cheaper GPUs with decent yields. You can't, however, put 3 or 4 on one card due to thermal and size restrictions.
There are clear limitations of dual GPU boards that I mentioned above. Scaling issues (performance), size ($$$ and compatibility), PCB complexity ($$$), mirroring VRAM ($$$). This is a solution that is only worth it under certain conditions.
If you need more performance, you can always buy 2-3-4 cards. Here's your idea of multi-GPU in use.
In this case 595 is more of an image product for Nvidia to fight for the single card performance crown...