Avoiding the burden of a "widget tree" that duplicates application state in complicated, inefficient and error-prone ways is more or less the whole point of reconstructing the whole GUI every frame from data structures that deserve to exist.
Agreed, but in traditional UI toolkits this burden is on the user-side. If the widget tree is hidden away inside the library it becomes just an internal implementation detail, a caching structure which might help make some things more efficient. Proof is in the pudding of course.
