You see this shit SO much more often than you would think. And the infuriating thing is, it seems to be most common among programs that are INCREDIBLY complex and sophisticated.
It'll be like this:
"What does my program do? Glad you asked. It simulates stress patterns in glass and ceramics, after they come out of a kiln. You can specify any melting temperature, adjust the composition of elements in the glass, and the ambient temperature of the cooling and tempering stages."
"Wow, can you show me how it works?"
"Sure! "
"O-oh. Do you have any plans to add a graphical user interface?"
"HAHAHAHAHHA, no. That's never happening. And here I thought you were serious about using advanced software, and being an intelligent person."
Obviously, that last part is just kinda implied. But sometimes, when users request a GUI, the goddamn developer will kinda get in their face, like that.
They always fall back on the position of "well, I developed this shit for free, for your ungrateful ass. So you can build your own fucking GUI."
But the thing about that is...no. And fuck you. I shouldn't have to be two-thirds of a fucking developer, in order to use the fucking software.
If you can figure out how to simulate molecules, or draw 3D stereograms, or translate hieroglyphics, or any other RIDICULOUSLY COMPLICATED SHIT, making a graphical user interface should be nothing to you. You should be able to do it in a fucking afternoon.
IT DEFINITELY SHOULD BE THE EASY PART, FOR YOU.
All the rest of us, who aren't programmers? We envy programmers, and their ability to really connect with computers, on that deep logic level.
If we could do that shit, we would. But a lot of us have tried, and we realize it's not a good use of our time. We can do cool stuff with software, but it's just not ever going to be worthwhile for us to struggle through the act of creating software.
Also, I hasten to add that I have put in my time, using command line interfaces. I used DOS, I used BBS systems, I have used modern command-line-only programs. I know how to do it, but I DON'T WANT TO.
I don't want to have to memorize commands. I don't consider a GUI workflow to be some kind of weird luxury. It has been a basic part of modern software, for around 40 years at this point. Literally get with the program, guys.
If you're serious about making software, get your shit together and implement a fucking GUI from the very first release. Nobody ought to be taking you seriously, if you refuse.
Game engines have their own tools and languages, which can be very different from non-game software, and needless to say require a completely different skill set from your average software without a GUI.
Most of the time, they cannot easily interoperate with the languages people use for other things. When you are building a game, you will be using the engine's tools and language from the very start, but porting an existing software to work inside of a game engine is unrealistic, and building normal software inside of a game engine would be completely absurd for most cases, both for performance reasons and also for developer convenience.
In theory you might be able to pack the original program on its own with no changes and just make the GUI interact with the actual program, but at that point it's already a completely separate project from the original software - a project that the original developer likely has no interest in, assuming that the original program already fulfills their own needs.
In other words: While it is possible to use Godot and alike to create a GUI, for most cases you would have to either do some extremely complex things to run the original program inside of the engine or (re)write the entire program from scratch inside of the engine, and odds are the engine will not have direct equivalents of third-party tools the program relies on.