Because
no one knows the truth + everyone sees the world in their own way. So there are no people who can be above or below someone else. Everyone is different by default. That is, it does not matter how old someone is, what position they have, what capital they have, etc. – by right of birth, all people are equal to each other.
(This does not apply to skills, resources, etc. If someone runs faster than another person, their skill can be valued higher. But these people are equal to each other in terms of their significance.)While the phrase “adults must be respected” breaks the essence of “respect” by default. All the cases where I have seen it used, in my view, were manipulations in which adults tried to take away the right to choose / have a voice or personal opinion from someone younger.