Because children are extra gullible. Children will happily sing a catchy tune from a non-interactive, non-personalized TV or radio commercial, even repeating marketing slogans they don't understand. You can win a kid's trust by giving him a candy. Have you ever seen a kid that has to be dragged away from a shop display, kicking, screaming and crying ?
Commercial social media are refined to the equivalent of advertisement cocaine. They are also a con man's wet dream (I refer you to numerous articles about Facebook frauds). It's even more than that, because it's the only drug which changes shape to more effectively get you. You have to be constantly vigilant and watch for new manipulation methods.
Parents watch over children not just because they like them, but because children are not capable of consistently making informed, rational decisions and weighting pros and cons.
I think parents themselves are not adult enough to deal with Facebook and avoid being manipulated or cheated. I think the world hasn't caught up with addictiveness of social media yet. Other drugs are already limited, for example by age. Netherlands doesn't limit access to drugs very much, instead it focuses on informational campaigns and rehab centers, with pretty good results. FB gives people something core to human nature - social contact, so addicts will defend it to the death. Age requirement wouldn't fly, so Netherlands-style approach is the only option left. At least until FB successfully lobbies to ban them or sues for defamation.
The dangers posed by social media are not inherently new, but fraudsters in meatspace have a harder time being completely anonymous, there are witnesses everywhere, your personal profile is not written on your back, and offenders risk local ostracism. FB is more like a private nightclub than real world with laws, courts and police. FB makes it easy to be a creep remotely and anonymously.