Tree climbing is a place to be, as opposed to a thing to do. It's different from rock climbing where one tries to overcome the rock or one's fears.
Tree climbing is safer too.
TCI climbers have only experienced blistered hands on the injury list over the fourteen years of TCI existence. Many have died on the rocks. There's a calming experience found in a tree quite unlike anything found on rocky earth. It's hard to describe. It attracts a different kind of person.