Because every other child you see has NEVER BEEN an adult. However, EVERY adult you see has once been a child. They all made it through childhood--they all survived.
If I am traveling to the jungle, I want to talk to someone who has been through the jungle. I want to know which snakes are poisonous, what to bring, how to climb...the adults in your life are helping you get ready for your trip to being an adult.
That's why you must respect adults.