Why as a civilian would you not require the same firepower that is available to police?
When shooting at a human target most people - including police - don't hit the person on the first shot.
When they do, they don't halt the threat with one round.
When they do halt the threat it often requires more than one round.
Conditions are damn near never optimum. It's never going to perfect light, perfect position, non-moving target, non-covered target, single target, no wind, no rain, you have your glasses on, your shooting hand is free and clear, you weren't awaken from a deep sleep by an alarm or breaking glass, there is no one behind the person you are shooting at, the person is within perfect range and is not shooting at you, etc. You can train for all that and more. Many of us do. Training teaches you that it is nothing like television. Unfortunately a great deal of the population doesn't understand that.
(A little 17 year old girl was shot three times with a .45 caliber. One of the rounds hit her in the chest. Physically, she is going to be OK. Taking three rounds from a .45 cal didn't stop her from crawling inside and calling for help. There is a long held myth that a 45. cal is a man stopper and that one round anywhere will knock a man down. That isn't true.
About a year ago a woman defending herself and her 3 children in their home, mid-day, good neighborhood, middle of the week from a dirt bag who picked their home at random to rob shot and hit the guy 5 times in the head a close range with rounds from a .38 caliber revolver. The guy was still able to make it back down 3 flights of stairs, out the front door and down the street in his car until he lost it by hitting a lamp post.
Depending on the type of revolver she was using she may have had only 5 rounds, she only had 6 at the most. Unless she had speed loaders or strip clips ready and was adept at reloading under extreme stress the criminal was very close to being able to harm her or her children even AFTER he had been hit 5 times in the head.)
Bad people who would do you harm often do not commit their crimes by themselves.
While preferable, often escape from the threat is not a viable option.
Depending on the drugs a drug addled maniac often doesn't respond to severe wounding in the way that normal humans do.
Here is the most critical point IMHO, most people who find themselves in a situation where they must defend themselves have none of this information about their assailants before hand. You do not know what it is going to take to stop the threat before you are already involved in attempting to do so.
When people are seriously threatened the first contact they make is to call for someone with a gun.
When seconds count police are minutes away.