Parents are human. They have limited emotional and mental reserves. Sometimes, a parent uses the phrase simply because they are tapped out. They have explained the "why" ten times already, or they are stressed about work, or they are overwhelmed. In these moments, using the phrase is an act of self-preservation. It protects the parent’s mental health, which is a resource the child also needs the parent to have.
It wasn’t until the 20th century, with the work of psychologists like Jean Piaget and Dr. Benjamin Spock, that the idea of the "child as a developing psyche" took hold. Suddenly, explaining why became more important than enforcing that . Because I Said So