C: Why isn't size_t a C keyword?

Question

sizeof is a C keyword. It returns the size in a type named size_t. However, size_t is not a keyword, but is

Answer 1

The simple reason is because it is not a fundamental type. If you look up the C standard you will find that fundamental types include int, char etc but not size_t. Why so? As others have already pointed out, size_t is an implementation specific type (i.e. a type capable of holding the size in number of "C bytes" of any object).

On the other hand, sizeof is an (unary) operator. All operators are keywords.