That may be largely due to massive amount of work put into the old way. Even if GA is just fundamentally a better idea for doing graphics, it doesn't have decades of software and hardware development to back it up.

Imagine selling this: I've got a new idea, with a mere re-education campaign for our developers, major graphics engine overhaul, and... convincing NVIDIA or AMD to implement some GA specific optimizations in hardware. We can have a much more mathematically satisfying code base!