How much longer would the C specification be if it were fully formalized? And even then, a formalization isn't enough: it has to be useful for actually proving

How much longer would the C specification be if it were fully formalized? And even then, a formalization isn't enough: it has to be useful for actually proving theorems, e.g. “such and such pattern is portable across conforming implementations”, and the size of the proof better not be much larger than the size of the program itself. If the C standard is so useful, try programming using the C standard as your sole debugging tool.