I don't get it. Let's say you create a game.exe, a windows application with a single window. User double clicks on game.exe icon on their desktop, and the window opens. User double clicks the icon again, and the game window opens again. If the developer want to prevent two instances of game.exe running at once, they need to actively detect and prevent this behaviour.

Just like you can start vim several times you can start gvim several times and have a few windows. This is the default behavior, and if the developer wants to change this for some reason, they need to put in some effort.

Maybe macOS is special? I'm not familiar with this platform.