Real life firearms are NOTHING like those on TV. First, your chances of actually hitting something from any non-trivial distance isn't that great. Even 10 yards is far from a sure hit, and just hitting doesn't necessarily count for much.
Second, the fact that the bad guy took a hit doesn't mean he's going down. In fact, he almost certainly will not. Anything less than a headshot destroying the brain (sorry to be graphic) may well have them still shooting back at you, even if the wound will ultimately prove fatal. Even severing the femoral artery or aorta isn't going to have the bad guy dropping in his tracks.
And in such a situation, your goal is to stop the threat. As you can see, ensuring that the treat is stopped requires a lot more than a double-tap.
In the UK they manage to subdue dangerous suspects without unloading the entire clip.