I think its so children are open to eating it after the parents have bought it. If the food doesn't look appealing the parent will have to work harder but its the parents making the food choices not the child. Most of the time children don't even go to the grocery store with their parents.
Tell me you have never had kids without telling me you’ve never had kids.
So parents buy unhealthy food for their children without any prompting, and the kids need to have their minds opened to it by advertising, otherwise they wouldn't it?