Task bar items don't represent exe files, they represent windows spawned by process started from exe files. Excel has many different behaviors depending on how you open a file. In fact, you can get excel into a state where double clicking a file opens in a hidden instance of excel that you can never see.

If you drag a file to a notepad window taskbar item, should it open in that window? should it open a new window in that process? Should it start a new process with that file? Should users have control over how that happens or should application developers? Or both? Or Microsoft?

On macos, theres only ever one instance of an application open so these questions are easy answer. On windows, they arent.