We've hit a point of diminishing returns with current graphics. We are very good at approximating the majority of real world effects at almost no cost to performance. But getting those last few effects is what's hard right now. Ray tracing is not only more computationally intensive, it also just hasn't had that much research done for optimising it for performance. All the ray tracing engines you see in 3D video production software are optimised for accuracy and flexibility. We don't just need more powerful hardware, we also need to figure out how to better utilise it for these new techniques, which takes time.