These are some pretty bold statements.
* "They wouldn't exist in the first place" implies that ImGui was the primary reason and foundation for creating these programs. As if using the traditional retained mode GUI is so unbearable that without ImGui the authors would have abandoned the idea of creating these tools in the first place.
* "Or wouldn't be as full-featured" implies that ImGui is either more full-featured or (if you meant time) is faster to develop with compared to a traditional retained mode GUI.
> I'm surprised this is so hard to comprehend?
Well, I'm surprised that some people keep presenting the immediate mode GUI as the silver-bullet alternative to the traditional GUI. Don't get me wrong: I understand that IMGUI is a great tool if you need to quickly add a throway GUI to a game, but otherwise there is a price to pay, both by the developer and the end user.