Honestly, I think the major reason is that kids need to learn not to bump into things, and if you slap a helmet on them, there's really the question of "so when do you take it off?"

It's different from say a bike or ski helmet, where you can rapidly seriously injure yourself. But Mac's pediatrician has a whole category of what she calls "coffee-table injuries," and to a certain extent they're part of the natural learning process.