Transferring my comment to an answer, because it makes logical sense. However, please note that this is unfounded conjecture. The actual reasoning of why the spec is written this way is still, technically, unknown.
Element height is defined by the height of the
children. If an element has padding-top: 10% (relative to parent
height), that is going to affect the height of the parent. Since the
height of the child is dependent on the height of the parent, and the
height of the parent is dependent on the height of the child, we’ll
either have inaccurate height, or an infinite loop. Sure, this only
affects the case where offset parent === parent, but still. It’s an
odd case that is difficult to resolve.
Update: The last couple sentences may not be entirely accurate. The height of the leaf element (child with no children) has an effect on the height of all elements above it, so this affects many different situations.