If it is so simple, answer the question. Why DO we like to sit in trees? It baffles me. I am and always have been afraid of heights for as long back as I can remember, but tree-houses were still cool when I was younger.
Tree houses are cool because it provides sanctuary from the outside world. Your parents do not govern you, because it is yours. You make the rules, you decide who can stay and who must leave. It is your own space, to do with what you please.

Climbing trees was fun because it provides a different view of things. Because you can look down on the rest of the world as if you are better than them.