The game runs well enough so the people who could optimize things by rewriting them from CPU to GPU are doing other things instead. Later performance is a noticeable problem to the dev team, from customer feedback and need to ship in more resource constrained environments (VR and XBox) and that person then can do work to improve performance.
It's also handy to have a reference CPU implementation both to get your head around a problem and because debugging on the GPU is extremely painful.
To go further down the rabbit hole it could be that they were resource constrained on the GPU and couldn't shift work there until other optimizations had been made. And so on with dependencies to getting a piece of work done on a complex project.