Brain tissue needs to optimize for a lot of things other than computational efficiency. It needs to stay operational for decades with minimal replacement of par

Brain tissue needs to optimize for a lot of things other than computational efficiency. It needs to stay operational for decades with minimal replacement of parts and it needs to be resilient to a fair amount of bumps, diseases and chemical injury. Silicon chips don't have to be build to survive these conditions so it's possible they can be much more efficient at the computational aspect.