Short answer: because no one is stopping the airline from doing it this way. This method gives them maximum flexibility, and they like it.
Just FYI, the UK *does* have such a law, which mandates that all children 12 or under be seated "within arm's reach" of their accompanying adult. The reasoning for the UK law is that, in the event of an emergency evacuation, panicked parents will cause delays trying to go against the flow to get to their children, thereby creating a considerable safety hazard. The law aims to prevent that. (BTW, in test evacuations in the UK, the airlines found that it was always the parents who caused the problem. The children, having been conditioned by school fire drills, exited the plane exactly as instructed, and did not try to find their parents until they got outside.)
When this topic has come up here in the past, I have suggested that concerned parents should write to their Congressional representative and to members of the House Aviation Subcommittee (
http://transportation.house.gov/aviation/index.shtml). I should point out that a new facet of debate almost always ensues from the moment I do this. There are several posters on the DIS who strongly believe that passing such a law would be a bad idea, on the grounds that it would adversely impact the ability of passengers flying in family groups to get tickets at the lowest fares. I'll concede that they might be right about that, though it has not been my experience when flying within the UK.