It is actually portable. It isn't 100% standard of course, and almost nothing is. Start with the fact that standard C is unable to have a GUI, unable to make a network connection, and unable to make a plain text editor that can respond immediately to any keystroke other than Enter.

You can't even implement the functions to provide those features. Any program using any of those features is fully undefined by the C standard. Any interesting program is thus undefined by standard C.