You appear to be conflating multiple windows in a single process with multiple processes. The default is that each process is almost entirely isolated from the rest of the running system. You have to actively work to even detect that another instance of the program you wrote is running.

The developer (of the app or the app framework) does have to deliberately implement "merging" of a second launched process. The default would be for the two processes to run side by side in both Windows and Linux. It has been that way forever.

The merging behaviour is also in principle possible for TUIs, btw, it's just that nobody does it. But with something like tmux is would be easy to set up that all subsequent calls to your binary actually connect to the already open session launched by the first instance.

I did not claim it (another user did), but I think your reaction captures the overall sentiment ITT, which is taking perceived status quo as some fundamental thing and defending the worse solution instead of changing the status quo. If all that energy went into perfecting GUI apps the way you want them to work, the ecosystem would have been much healthier. I've seen it in Electron already, and TUI apps are headed the same way, it's simply inevitable. The JS and Python bloat and fragmentation of conventions are already there, reinventing things that used to be simple in a complex way is next (in a way it already happened, e.g. a ton of basic things like hotkey schemes have to be reinvented from scratch each time in TUIs).