A driver's reaction time is not fixed. Some things make it longer, so the driver reacts more slowly to a hazard. A longer reaction time increases the thinking distance, and so the stopping distance. Thinking distance = speed × reaction time, so a car at 20 m/s with a reaction time of 0.2 s has a thinking distance of 4 m, but with a reaction time of 0.6 s it is 12 m.
Drugs are one cause. Alcohol slows the brain's response, and some other drugs, including some medicines that cause drowsiness, have a similar effect. Tiredness also lengthens reaction time. Drugs and tiredness make the driver slower at responding even when they are looking at the road.
Distractions are another cause. Using a mobile phone, talking to passengers, adjusting the radio or loud music all take the driver's attention away from the road. The driver notices the hazard later, so the vehicle travels further before the brakes are applied. This is why drink driving and using a phone while driving are dangerous.