I don't think it is exactly TUI vs GUI, but more keyboard-first vs mouse-first, i.e. how you interact with it. Granted, Desktop GUI are usually developed mouse-first, but it doesn't have to be that way: if you take windows 98 / windows 2000, you could get everything done more efficiently with a keyboard than with a mouse.
Just think about the difference between an app developed specifically for the desktop (mouse) or for mobile (touch): same GUI but completely different programs and experience
Plus responsiveness. Terminal apps have faster response time than GUIs. And when someone is typing like 10 chars per seconds it matters.
The usecase is different between terminal vs GUI. Where in GUI you look - choose - decide and click. And in terminal you blindly storm on the keyboard as fast as you can.
Terminal apps are running under a GUI terminal for at least two decades now, there isn't really a reason for a GUI application to not be as responsive as a terminal application, it is just that "modern" GUI toolkits being slow.
IMO a reason is because nobody seems to bat an eye when faced with a "TUI" made out of lines, boxes and garish colors but if a GUI is made like that it is suddenly ugly so toolkit developers put a ton of effort into shiny looks.
But try a GUI made with Win32, Motif, Tk (not Ttk) or even Gtk1 and it'll be as responsive as any terminal application (of course it also depends on the application, there isn't much the GUI can do if the application abuses it or is sluggish for other reasons ).
A bit of that is also related to different UX design, with many of the venerated old TUIs actually having better UX, not due to being TUI, but because they avoided certain popular (anti-)patterns of GUI programming.
A top example, from discussing with non-techies who had the tools change from TUI to GUI, is the drop-down menu bar at the top of the window (or top of the screen, for mac). What used to be few keys at worst, turned out to multiple long trips by mouse to select an option 3+ levels of nesting deep, because the job was just that complex - but there was no contextual shortcut available.
Many old TUIs (and we're talking divergent brother of S/370 with random other stuff in the mix old) tended to have ways to reach pretty deep in the navigation structure pretty fast (sometimes even direct - for example airline reservation systems, where you often can make a CLI command go straight to a completely different context).
> The usecase is different between terminal vs GUI. Where in GUI you look - choose - decide and click. And in terminal you blindly storm on the keyboard as fast as you can.
Actually I think it is more nuanced than that.
Some GUI programs are well optimized for fast keyboard only usage even if you can use point and click. NetBeans, Eclipse, IntelliJ, Sublime Text and VS Code are on top of my mind but even some old web applications used to be quite usable I think.
Just think about the difference between an app developed specifically for the desktop (mouse) or for mobile (touch): same GUI but completely different programs and experience