You can open multiple Excel instances since at least Excel 2003 but Excel would not go out of its way to open files in a new instance if you just double click on the file.
Excel doesn't allow you to do that, even though the code clearly supports it. (As others have said, you can launch multiple copies with their own windows by running the .exe manually.) As far as I can tell, it's just a matter of someone at MS being an asshat.
In any case a tab-based Web browser is a completely different sort of application than Excel.