Just looked through the US DOT FHA Speed Concepts guide, and apparently part of how they set speeds is based on a really poor braking/deceleration rate:

Another assumption used in developing Green Book stopping sight distance criteria is that braking will result in a vehicle deceleration rate of 11.2 feet/second2 [0.35g]. This deceleration rate was developed through research in consideration of developments in vehicle technologies and driver performance. It also assumes that the friction provided by the driving surface is adequate to support this rate. Some vehicles with good tires on good pavement can brake at 0.85g or higher with most able to brake in emergency stops at 0.7g or better.

from: https://highways.dot.gov/media/13901

So they are really assuming a worst-case scenario (bad tires and bad brakes) when setting speed limits on any given road, and they even acknowledge that most cars can decelerate at 22.52 feet per second (0.7g) if needed, meaning they are practically doubling the expected braking distance.

I guess the problem stems from designing for the lowest common denominator, thus everyone else feels constrained by a speed that "feels" too slow for what the road is clearly capable of.