Why can't a pointer of a "struct with const member" type point to a "struct with non-const member"?

This code doesn't compile:

struct s_t {
    int a;
};

struct c_s_t {
    const int a;
};

s_t s;
c_s_t *c_s = &s;

ibug@ubuntu:~ $ g++ -fsyntax-only t.cpp
t.cpp:10:15: error: cannot convert ‘s_t*’ to ‘c_s_t*’ in initialization
 c_s_t *c_s = &s;
               ^

However this one compiles perfectly:

int a, *pa = &a, **ppa = &pa, ***pppa = &ppa;
const int * const * const * const cpcpcpa = pppa;

I understand that a pointer that is more CV-qualified at any level can point to a less CV-qualified object at any level, but why isn't it the same for structures?

---

The above problem statement is a MCVE of a more complex problem, where my friend was trying to convert pointers between t_s_t<T> and t_s_t<const T>, where t_s_t is a template struct type with one template parameter typename, and T is an arbitrary type.

The reason is that s_t and c_s_t are different types.

Even if you define c_s_t as:

struct c_s_t {
    int a; // <-- non-const!
};

Then:

s_t s;
c_s_t *c_s = &s;

It is still not going to work.