That is an interesting question, which leads me to believe that there was some form of reasoning behind the practice. Can't figure out now what exactly it was, but I'm assuming there was some kind of reasoning behind the awful practice. People usually don't engage in horrible practices or conjured up beliefs unless they are demented, which they might have been, and/or considering some form of worship.
Plus it seems that the Almighty God, although He thought it was horrible that the Israelites practiced this, He did not miraculously step in to stop it, but it continued for a while.