They're not const because of backwards compatibility. Const correctness in C is a lot weaker than the way C++ enforces it, letting you implicitly cast it away in a lot of cases.
Quote from gcc manual, explaining why you need to compile old code with -writable-strings option: "you cannot call mktemp with a string constant argument. The function mktemp always alters the string its argument points to.
Another consequence is that sscanf does not work on some systems when passed a string constant as its format control string or input. This is because sscanf incorrectly tries to write into the string constant. Likewise fscanf and scanf."