I think it's because data is often spread out between both layers, meaning that even if you could rip a game to two disks, you may stand a chance that data might not end up on the disk that requires it, leading the game to be unbootable.
I'm sure there's a technical explanation for what I'm on about, but I'll be damned if I know it. I'm not even sure if I'm right, but it sounds close enough for Government work.