The vapor sensor isn’t 100% infallible. In most states now, holding the handle cannot be done with a device, switch, lock, lever, etc. Also, it’s an issue of user error. Let’s say the gas tank is full and the nozzle shuts off. The user thinks it’s not full, and locks the lever again. The tank is now merrily overflowing, but the handle is in a locked position. Or, in the previous scenario, the handle locks accidentally.
Vapor sensor failure most often occurs when you are filling up a gas can or when the vapor sensor is under liquid. In these situations, it can be very dangerous to lock the handle.
On top of all this, gasoline is not a safe substance. It can irritate eyes and skin, and even if it doesn’t feel wet, evaporating gas is very, very flammable. For example, there’s numerous stories of people who spilled gas on their pants, lit a cigarette, and set themselves on fire. The vapors from the pants caught fire even though the source of fire was relatively far away.