@thor Because hierarchies are a horrible way to arrange objects of the same rank. These should be tags, not folders, adding folders to Springboard was a mistake — this is where the UI of iOS went wrong: "Why are we thinking about this shish, let's make the user arrange this mess of apps himself!"
I think they've fixed it a couple of years ago, now apps are arranged into categories automatically and one app can belong to more than one category — at least that's what I've seen on my mom's iPad.