Probably because kids are dumb and have a tendency to play with guns when they find them. They've been known to either blow their own heads off or shoot their friends. I didn't do either of those things, even though I loved gun violence in my movies/cartoons and I lived in a house full of guns. When I was a little kid, my dad took me aside and said "This is a gun. It can kill you. Don't touch it."
Of course, many people don't take the time to do this. Recently in the US, there has been an epidemic of senseless child deaths from TVs falling on kids. This is because people are getting rid of their giant old tube TVs and poor people who want something for nothing set them up in their houses on the most rickety unstable thing available. This has led to morning news shows having detailed discussions about preventative measures so TVs don't fall on kids. Never mind the thought that the individuals who have let that happen were so lazy, cheap, and neglectful that they wouldn't even pay any attention to such a warning regardless.
Likewise, gun owners who don't tell their kids about gun safety are rolling the dice on their lives. If it's not one neglectful thing that will kill your kid, it will be another. I can't count the number of times I've seen small children run through a parking lot.
But hey, it's gotta be that damned children's media, glamorizing running through parking lots, playing with guns, and encouraging kids to shake the 70 pound television. I mean hey, the anti-drug cartoons really worked great on my generation.