I don't understand what you're responding to.

TUI apps are multi-instance by default since the user can invoke the program in different pty sessions. The developer would have to actively fight that.

GUI SDKs generally require the developer to pick a solution. And it's easier to default to the simpler solution where you have one instance, one data model, one window.

Whether things might change in the future isn't relevant because we're talking about how things are today, not speculating about the future. And the time to change one's opinion is when reality changes, not ahead of time based on how you think things hopefully will be.