I think you may be imagining what a human sees in a foggy camera image, rather than what a neural network can see in it. In fact, atmospheric obstructions degrade Lidar's signal much faster than a camera's.

If you check out NVidia's DriveNet demo from a couple years back, you'll see that they already have a vision-based NN that outperforms humans in fog and snow[1]. We can debate whether people should drive in those conditions at all, but today's expectations will be the baseline that SDVs are up against, and cameras are much better suited than Lidar to achieve that baseline.

[1] Start at 7:21 https://www.youtube.com/watch?v=HJ58dbd5g8g