This is something that I unfortunately have a "reasonable" explanation. It involves some historic idiosyncrasies and GNOME devs' decisions along time. If you want to listen to some (read an) interesting story here's some of it:

* First: GNOME was born without an IDE. But, there was an interesting little project (by Naba Kumar) which started in mid to late 90's called Anjuta. IIRC, Anjuta 0.x to 1.x was what was called a "monolithic" IDE; that is: it didn't had any plugins and its features were "hard-coded". The 2.0 rewrite allowed it to have plugins, but they didn't covered (at first) all of the features of Anjuta 1.x. It was only around 2.4 or 2.6 that Anjuta finally got feature-rich and reasonably stable enough to become part of the GNOME project and the "IDE of the GNOME project" although most GNOME devs didn't use it.

* Second: GNOME was born without a GUI Builder. The most successful one was Glade. At first, glade generated C source code for the UI using GTK. Then people thought it was bad and someone created libglade. The 'project libglade' was two things: a XML format specifying an UI description and a library that could dynamically (at runtime) load it and build a GTK interface from that description. It could even connect signals without a single line of C source code.

Since libglade seemed like a good idea it was eventually incorporated into GTK in a format that is now called GtkBuilder.

Parallel to this, many events happened. Glade changed hands a few times and eventually its main developer became Chema Celorio. Celorio was a skydiver and one day he had a problem with his parachute and died. Glade development became dormant until it was picked up again by Tristan Van Berkom. Tristan modernized it, 'resurrected' it, improved its UI, rewrote many parts of it and eventually removed its code generation back-end to use only the GtkBuilder format.

* Third: At the same time, Anjuta development also changed hands. I remember Johannes Schimidt and a few others. At some point, there was an integration between Glade and Anjuta, but it was not very different from running the two tools separated except for the fact that they shared the same window.

There was some hope when, during the selection for Google Summer of code an interesting project was chosen: improve the integration by Pavel Kostyuchenko. The project was successful and the integration between Glade and Anjuta improved because of it.

I was happy and hopeful for these facts when one day I saw that (part of) the integration code was chopped off. I was so intrigued by this fact that I decided to ask Anjuta devs personally (on IRC) what had happened. The answer was something like: "it was not as good as we'd like, fixing it would be much more work than rewriting it."

That was when I decided to join the project to fix it myself. I improved the integration between Glade and Anjuta. Rewrote how it created functions inside the C source code, how it discovered automatically which .c and .h files were associated with the .ui file. Implemented automatic creation of members on the C struct that corresponded to the UI elements, fixed some bugs, wrote a documentation which was a good tutorial on how to use these features... I even implemented the preview feature in Glade. I really made the integration work to my liking. It was the workflow I dreamed.

The future seemed bright: there were new developers, frequent commits, new features were implemented frequently... But GTK evolved faster than Glade. Tristan eventually abandoned it and it was picked up by Juan Pablo Ugarte. By around this time, Glade became something of a monstrosity of technical debt, slow, crash-prone and buggy but it worked. Implementing support for modern GTK features (like bindings and css) seemed too much work and too hard to do.

* Fourth: Around 2013 Christian Hergert started developing a modern IDE for GNOME: GNOME Builder. He left his job, started a campaign to get money to back him up during first versions and quickly showed a lot of progress. It was said at the time that it was going to have Glade integration (it eventually did have, but it was never a decent one). The project thrived and Redhat hired him.

Development of Anjuta slowly became dormant after this. It is now archived.

Hergert started another project, Drafting, to replace Glade but it was de-prioritized. Ugarte basically abandoned Glade and started Cambalache. Cambalache seems like a good idea, works but has no IDE integration and is still mostly a "one man show".

And that is how we arrived at where we are. GNOME still lacks a good UI builder with IDE integration and no current plans to fill this gap.