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

size_t is not a keyword by necessity. Different architectures often have different sizes for integral types. For example a 64 bit machine is likely to have an unsigned long long as size_t if they didn't decide to make int a 64 bit datatype.

If you make sizeof a builtin type to the compiler, then it will take away the power to do cross compilation.

Also, sizeof is more like a magic compile time macro (think c++ template) which explains why it is a keyword instead of defined type.