Lets break some layers down in order to answer this question.
Games live on the software level, and must communicate down to the firmware level of the GPU to instruct what graphics to display on the screen. We understand that instructions are sent back and forth, but what are these instructions? Each game is built off an engine that has libraries to interact with the drivers. The drives, than communicate down to the firmware, and thus the hardware itself. Each game must be built around a library of available commands that are first available at the firmware level, but trimmed as we move up to the game software level. Not all firmware instructions are available, because the developers will decide which are most important.
Given this break down, we can see that the SLI/XFIRE instructions would be available up to the game software, if allowed. This is what most people understand, developers must enable multi-gpu rendering, and they can use even more instructions to smooth out performance. However, a driver can recognize specific instructions and instruct the firmware to act differently, this is where driver optimization comes into play and why AMD and Nvidia make sure to stay on top of driver support. Game developers cannot always keep on top of the latest and greatest, because they are more worried about the game running on a variety of hardware. Driver support relieves some of the effort of the game developers, while also creating a customer support role (a reason as to why Nvidia is getting a higher market share atm).