It's all about the pressure that the gas pumps are designed to feel, essentially. The gas pump is made so that it can cleverly tell how much air is flowing through the nozzle. That's why it works when you've got an empty gas tank because the pump can feel the air. Then, once you've filled the tank, the pump machine can feel the changes in the air pressure. It's at this point that your gas stops pumping. It's specifically designed like this so that it safely fills your tank and doesn't overflow. This would cause excess spillages and also be a fire hazard should the fumes create a spark.