My guess is that is because a TUI can’t give focus to the existing instance. When a GUI application realizes that an instance is already running, it can ask the windowing manager to unhide and focus the running instance.

For a TUI to do the same, it wouldn’t just need to find and focus the window, but would need to trace it through the myriad ways that a TUI can be displayed. I may run a TUI inside a docker container, as part of a tmux session, over ssh. If a TUI somehow managed to focus the existing instance through those layers of wrapping, I would be very surprised, disgusted, and impressed.

So I think it’s that the end goal of “redirect user to existing instance” is infeasible, so nobody bothers to enforce a single instance, since the rest of the steps aren’t possible.