Why do people say it's dangerous to leave your vehicle running when putting gas it?

Because it is an old myth that people still believe. “I saw it on Facebook so it has to be true” is more common than people want to admit.

Gas stations put up signs warning against filling with the motor running because people are plain stupid and they want to reduce their liability. They get out of a running car still in gear, it drives off and hits something. It’s also easier to steal a car if you are not in it and it’s running. Ignition systems do not have sparks flying out of them. Exhaust pipes do not have fire shooting out of them. In fact, the gasses coming out of the exhaust pipe are mainly CO2 and H2O. Both used in fire extinguishers.

There is a much greater chance of static electricity causing a fire than a running car. And that is even quite small.

Did you know there are open electrical connections on the fuel pump in the gas tank? They can do this because combustion only takes place under certain air to fuel ratios. The odds of this ratio happening in the open atmosphere is astronomical.

If there truly was a danger, gas pumps would not be so close together as they are. And you would have to push your your car to the pump while it’s not running.