Why does this trope exist in the first place if its not realistic?
Could it be because most writers & artists are ignorant of how bullets work? Yes & No. It certainly is true that many creators do not know the exact details of ballistic trauma, but other times it is because the work in question does not need it, or may even be impacted by realism, and others are simply because of rating. Probably the most common reason why this trope exists is because realistically portraying lethal & violent gunshots can be very unpleasant to the viewer (for lack of a better word), and depending on the genre would either be required, not be needed, or played somewhat unrealistically. This was especially the case in older films & TV shows where the depiction of violence was heavily regulated. But in the end, The Law of Conservation of Detail applies to this trope in various works.
For example, many Action films and especially Superhero films often show mooks & red-shirts die instantly after being shot simply because its typically not the focus of the scenes and because they want to maintain a PG-13 rating. In these kind of films, the spectacle of the action is the main focus, not the violence in question even when there is blood. Making the mooks' deaths relatively painless is a safe way of conveying the necessary level of fictional violence without ruffling the feathers of the audience or those who make the ratings; and if kids watch these movies, it's best to not be too extreme with gunplay. If having realistic gunplay is not a necessity to effectively convey what is happening in the work, then why include it? Especially if the target audience is not looking for excessive violence. Ultimately, such works generally don't need realistic gun-trauma and may actually be hampered by it.
On the other hand, films that are geared more towards realism and for a mature audience, its much more common to not rely on instant death bullets, or at least only using them when they are likely to happen or not. War films in particular rely on conveying the firearms and their effects on victims as realistically as possible and thus show how incredibly unpleasant and disturbing it is for anyone to get shot. The same applies to certain crime films, where the victims often die painfully from being shot. Having more realism to these scenes often serve to the work's benefit, and it would be incredibly odd if they took too many noticeable liberties in how the victim would react. It goes without saying, but many of the weaker war & crime films tend to rely heavily on Instant Death Bullets that show gunshots very unrealistically (along with a myriad of other problems that plague these films).
So ultimately, proper use of the Instant Death Bullet Cliche comes down to genre, along with what the situation requires for the artist and for the audience.
