I think it boils down to style and intent.
Style = Games are heavily stylised in most cases. So even if gore in games is high-fidelity, your brain knows it's "cartoon" on some level. Be it due to the unrealistic pixellation or rendering of the models, the unrealistic context of the action, the HUD on your screen, etc. You see blood/violence IRL and it's more uncanny because you're used to seeing real life as-is without this stuff in it, then it's there and you know it's real by the virtue of it being "out of place", as well as it being perfectly realised in your brain.
Intent = games typically don't imply any deeper malice in their violence. It's basically a mechanical exercise. You could replace shooting with bouncing balls off people's heads. You could replace the blood and dismemberment with daisies and paint and it would mechanically stay the same. In real life, seeing blood or dismemberment tells you something very grim about the universe we live in that your brain has to calculate as a real threat/risk. If you see someone being attacked or hurt, you know that either a person's true malicious intent caused it, or a nihilistic, uncaring universe did. In a videogame there's no such deeper meaning/intent behind violence (99.9% of the time).
There are few games that actually overcome these two factors - but I think we'll see more and more in future. I actually think visceral multiplayer action games tend to get the "intent" of violence more than other genres/styles.
I'll always remember when three other players in The Last of Us's Factions mode were chasing me. I ran out of a garage and stepped left behind the wall. The first two players came running out - they hadn't seen me, I'd lost them. They pressed ahead into another building. The third emerged from the garage and I grabbed him, tore into his throat with my shiv. I ran up behind the second, pulled my shotgun and blasted him in the back. Just as the third turned to face me I yanked him to the ground with my 2x4 with nails in it and then pressed "Execution" and watched my character pummel the person in the face with the end of the board until they were dead.
I was legitimately shaking after it happened. It felt like I'd really been hunted in real life and I'd truly, visciously murdered three people who were attacking me. This is one of the few instances of true "violence" I think I've ever experienced in a videogame. They had real malice for me, I had real malice for them, and the representation was just viscious and brutal enough to be like something you'd see in real life.
