If you're in traffic (i.e. if you are part of the traffic) and you leave a 3 second gap between you and the car in front of you, another car will drive into that gap. If you back off to create another 3 second gap, it will happen again. Even worse, if you hit the brakes to create that three second gap, even if it's very lightly, you might cause an even worse traffic jam behind you.

I would prefer to leave a big gap to the traffic in front of me, but in many cases 3 seconds simply isn't practical. A car merging into the lane in front of you is inherently more dangerous than a car already being in that lane. If you keep trying to maintain a 3 second gap in heavy traffic, not only do you put yourself in more danger as you keep having cars merging in front of you, you also cause more danger to the drivers behind you by constantly backing off or braking to try to maintain a gap.

It would be absolutely wonderful if everybody believed in the 3 second rule. Traffic would flow so much more smoothly. But, apparently that isn't human nature. And, if you keep fighting for that gap when nobody else believes in it, you can actually make things less safe for yourself and for others.