Because it's not a primitive. It's handled by a special case in the interpreter, (= f apply) (applyf (car args) (reduce join (cdr args)) a s r m) rather

Because it's not a primitive. It's handled by a special case in the interpreter, (= f apply) (applyf (car args) (reduce join (cdr args)) a s r m) rather than being something whose behavior you have to assume.