It's likely just a game mechanic to stop players getting frustrated and quitting the game within the first few minutes.
If the character took damage as they would in reality, then they would not get through many of the situations they can find themselves in during a game. I can think of several battles I've played in games that if it had been in reality anybody would be six foot under within seconds.
There has to be some leeway to a greater or lesser degree depending on what level you play on.
Because it’s a video game. If it was realistic and had the character die that quickly, then it’d be way more frustrating and it’d be unenjoyable, unless of course, the game is easy enough that most players never ever die.
Because most game didn't implement realism in their game, since it could drove lots of people away.
But that doesn't mean that realistic games didn't exist. If you want to play those kind of games, try play Arma series or Insurgency on PC, which implement some realism in their game (people die in 1-2 shots, gun physics, etc).