N.B.
There are universal stages that all children go through, such as puberty, learning to crawl, developmental stages, etc... Sometimes kids don't learn to crawl and later on they are missing some developmental traits. They've had to go back, as a school child or an adult, and learn to crawl. These stages are developmentally crucial.
Playing Peek A Boo is a sign of the Object Permanence stage. So you might think playing peek a boo is a silly thing for a kid's development to be gauged on but Object Permanence a universal stage that all kids need AND normally go through.
So it appears that you haven't studied child development and think that years and years of professionals that have studied child development are less "smart" than you. I don't mean to be mean but you really should do some research before blasting something that is absolutely based on fact with years and years of professional research across the world then put together by people who do know the stages of child development. They do know what they're talking about, ALL kids do through these stages and if a child is missing one of these stages they know what to do to teach them and get them to go through it.