Generally speaking, you want to tie a rope as low as possible to a tree, not as high as your length allows. Not only because the base of a tree is its sturdiest portion above ground, but because it avoids the totally unnecessary lever force stressing the tree. I'd say he tied it so high because he was really worried about the rope getting close to the toxic water. In any case, the rope is far more likely to simply snap, even under its own weight, than to become loose because of the anchor breaking. Even in our world, with advanced material engineering based on science and long industrial knowledge/traditions, it's challenging to make elevators for the tallest buildings. And they even aren't a kilometer tall so far. Spirit vines must be so nice.