For the same reason modern movies recorded digitally still have film grain. They are added to convey something beyond what the camera captures. Is that always effective or done meaningfully? Definitely not. But there is something to it... It shouldn't be seen as any different than color grading. Every movie has color grading. As a result, they rarely ever capture reality as-is. It would look like a documentary or soap-opera if 1:1 replication and maximum IQ was always the goal. It would be objectively better, but less effective in its subjective impact. It loses visual identity and every movie would start looking the same.
Criticizing the very existence of artificial flaws in movies/games is like criticizing distortion in a guitar solo. Those are artificial flaws too. Perfection doesn't have to be the goal. That's not the purpose of art.
I actually think this shot would have been more potent with
more film grain. Not less. It still looks too clean and a little bit at odds with the scene's context. It's just not messy enough. Purely subjective opinion.
The "eye" argument is not valid either. First person games are just PoV cameras. Rarely eye simulation.
I do agree there is something to be said about when these defects are effective and when it is not. But I do believe it's all subjective. Your appeal to logic isn't as strong an argument as you believe. There is not much logic either way, just like we can't actually see the veins in our eyes when we are low on health. All of it is make-believe. Some of it just more effective than others. And finding that right balance is the job of an artist. Some are simply better at it than others.