[CyberDildonics]

most GUIs by necessity involve fairly deep hierarchies of graphical objects

First, this isn't really true. You might typically have a window, a container, a layout object and then your gui components.

Second you don't need nested calls, you just add one component to another.

or you're going to scatter the fragments across a number of files

Why would that be true?

they need to be reassembled in the reader's head in order to understand what's going on.

This is a bizarre way to make a GUI let alone thinking it's necessary. Where is this idea coming from?

FLTK, JUCE, Tk, ImGUI, Swing and Qt are not like this at all.