In this case, history is a factor. C defined these inputs as “not constant”, and compatibility with (a good portion of) existing C code was an early goal of C++.
Some UNIX APIs, such as getopt, actually do manipulate argv[], so it can’t be made const for that reason also.
(Aside: Interestingly, although getopt‘s prototype suggests it won’t modify argv[] but may modify the strings pointed to, the Linux man page indicates that getopt permutes its arguments, and it appears they know they’re being naughty. The man page at the Open Group does not mention this permutation.)
Putting const on argc and argv wouldn’t buy much, and it would invalidate some old-school programming practices, such as:
// print out all the arguments:
while (--argc)
std::cout << *++argv << std::endl;
I’ve written such programs in C, and I know I’m not alone. I copied the example from somewhere.