It is Pakistan, drivers set their own norms here. If we had speed traps/cameras and patrolling on highways, only then can max speed limits be enforced. I agree with your assessment of stopping distance and safety but please don't think for an instance that Pakistan is the perfect place when it comes to traffic rules. You drive right lane on KH, you drive in excess of 100 kph at least. That's that.
And that's not to justify myself as a speed demon. I only drive fast when the coast is clear. My normal commuting times on KH are fairly away from rush hour.