Because clicking a mouse to go pew pew at fictional characters is drastically different than pointing and shooting a gun at a human being.

Even the most realistic military shooters, you don't just get a red tint over your eyes if you get shot, you can't wait it out or use a medkit to immediately be fully recovered, and people don't respawn the next match after they are killed. They don't show how gruesome and nerve-wracking real violence it is. They can't show the lasting consequences of that. People who play video games might not even know how heavy a real gun is.

And then there are things like Fortnite and Overwatch, which are just silly cartoons. No comparison.