Honestly, when you're driving, you sometimes only see the car in front of you. You might see it slowing down first, and you start slowing down, or continue because there is enough distance anyway. Then the car in front of you stops abruptly or crashes. Now you panic and brake hard. Maybe you still crash into the car in front. Even if you stop before crashing, this hard braking causes the car behind you to crash into you.
TL;DR - Sometimes the 3 second following distance is just not enough even if someone is paying attention because they can only see the car in front of them.
TL;DR - Sometimes the 3 second following distance is just not enough even if someone is paying attention because they can only see the car in front of them.