[CyberDildonics]

It's not difficult or disorganized to layout GUIs in a programming language. HTML exists as an alternative to having nothing. If you have a programming language you can give the data to the GUI library directly without having to learn a new markup language, have the bloat of a new markup language or learn the quirks a new markup language. You can also put format it and put newlines in there.

[mvc]

The thing is, most GUIs by necessity involve fairly deep hierarchies of graphical objects so you're either going to have deeply nested calls like this, or you're going to scatter the fragments across a number of files in a way that they need to be reassembled in the reader's head in order to understand what's going on.

[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.