Unfortunately, in a lot of areas, speed limits are more about revenue than safety. I've rarely seen reckless speeders targeted in conditions where safety is a very real concern, such as during poor weather conditions or bad roads. Most of the time, it's areas of long straights under good conditions where they'll set up shop, because the majority of motorists are comfortable in going somewhat faster than 55.
Speed limits need to be in place to keep things in check, but realistic limits are rare. It's one thing to set things up to the lowest common denominator, but in the real world, driving at those specified limits all the time probably creates more of a hazard than going at a higher speed but with the flow. It's the speed differential that plays a factor in creating dangerous situations as well as just an arbitrary speed. It's more convenient to have a fixed limit, because it would hold up better in court, than attempting to prove a motorist was going some amount faster than average.
Not all roads have the same capacity to handle traffic at any given speed, but basing speed limits on that may prove too confusing to the average motorist, and eliminate a lucrative source of income, so there's not much incentive for that to change.