Because, at a certain age, watching a violent scene does not necessarily make a child have violent thoughts or violent tendencies. But kids will repeat things they hear people say. My sister had issues with my nephews doing that for a very long time indeed. She had to be very strict about it because they would laugh together and endlessly repeat it.
Similarly with sex… I watched the Tim Burton Batman film in the theatre when I was 11; but my parents would not let me watch Spaceballs. My mother said the reason why is because I was still developing my identity or my understanding of sexuality at the time, whereas she knew violence would not affect my developing mind the same way.
I don’t know, I was pretty desensitized to violence at a young age and that probably damaged me somehow. I didn’t ever do anything violent, but seeing some surprisingly gory violence in real life like in schoolyard fights and other kids’ bike crashes had no effect on me ever.