@WebsiteDeveloper 
I agree with you that it is more convenient, but it is not standard both on Windows and Mac. You can try it in the Open File dialog on both platforms. On Windows, default button switches as you tab between OK and Cancel button. So it is not ambiguous when you hit Enter key when focus is on Cancel button. However, if you tab to other buttons (Organize, New Folder, More Options) and hit Enter key, you will see that default button takes action. On Mac, it is pretty clear that hitting Enter key on any button other than OK (default button) always takes the action of the default button. 

Regarding your claim of default behavior in chrome and firefox, I'm not sure you're referring to any modal dialog that has that behavior. Let me know if you see that behavior in any modal dialog and not in a web page.