why do some parents act like kids know better then them?

why do parents act like kids know what to do better then them? my mom let me do whatever I want and she thinks kids should be in charge and you should let them do whatever they want. I turned out fine now, but as a kid I panted my self and the walls instead of the paper, cut my hair and sat on the computer all day. I feel like its clear that parent should restrict their freedom until they show that they choose to use their time for a good purpose. its also a problem when your looking after someone elces kids one of my moms friends complains to me about my mom because she would let her friends kid do whatever they want and eat as much candy they want and now the kid does not like her mom, she likes my mom better, I would be unhappy about that too. my mom thinks that if the kid gets a stomach ache they will learn their lesson.