I think people tend to forget that the speed limit is set for general YEAR ROUND conditions. So on the highways & freeways, 100 is safe in rain - just don't tail gate! Whereas in summer on a bright clear day, 120 would be perfectly safe. So the powers that be set the safest average.
Which is dopey. There are plenty of roads in the world that have a different speed limit if the road is wet and people seem to cope. In fact if you can't cope with the concept of variable speed limits maybe you aren't clever enough to drive.