Why pointer to pointer of the same hierarchy are unrelated?

Question

Why does the following code produce error C2440: 'initializing' : cannot convert from 'Bar **' to 'Foo **'. Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast.

struct Foo
{
};

struct Bar : Foo
{
};

int main()
{
    Bar **ppb = nullptr;
    Foo **ppf = ppb;
    return 0;
}

Aren't Foo and Bar related, so upcasting a pointer to pointer from one to another should work?