[Animats]

The answer to that is in this hour-long SIGGRAPH video.[1] Some of the operations needed are not done well, or at all, by the GPU.

[1] https://www.youtube.com/watch?v=eviSykqSUUw

[janito]

Here's the relevant part of the (really cool!) video: https://www.youtube.com/watch?v=eviSykqSUUw&t=1888s