You seem to be confusing fonts with letters or characters (graphemes). A font is simply a collection of specific visual representations of graphemes, each of which should be readily understandable by any reader as a representation of a certain grapheme. An icon, on the other hand, has no strict tie to a grapheme (some might happen to be graphemes, but most aren't). Of course, Unicode, in all its vastness and splendor, has chosen to include lots of code points that wouldn't traditionally be considered graphemes, and fonts are expected to contain specific visual representations for those as well. So at the end of the day things are complicated.