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In C, because function return values, and qualifying values is meaningless.
It may be different in C++, check other answers.
const int i = (const int)42; /* meaningless, the 42 is never gonna change */
int const foo(void); /* meaningless, the value returned from foo is never gonna change */
Only objects can be meaningfully qualified.
const int *ip = (const int *)&errno; /* ok, `ip` points to an object qualified with `const` */
const char *foo(void); /* ok, `foo()` returns a pointer to a qualified object */