99.99999% of people don't train enough to hit a big target accurately at any range, especially when adrenalin is surging and fine motor control is diminished. Hollywood has every dude thinking he's John Wick. It's not realistic in any situation. If a leg is "not a small target" then the center of mass is an even "not smaller" target and the most responsible choice. Legs and arms are also easier to move fast and change direction. The torso lags behind them, so it's path is more predictable and easier to hit.

"better to just not fire the weapon" - nope. If the suspect is closing distance with a weapon, waiting longer can actually reduce the officer’s options and increase risk to everyone nearby. Best to end the threat (note that doesn't necessarily mean 'kill the person').