Why are arrays not assignable in C/C++?

Question

I understand that memberwise assignment of arrays is not supported, such that the following will not work:

int num1[3] = {1,2,3};
int num2[3];
num2 = num1; // "error: invalid array assignment"

I just accepted this as fact, figuring that the aim of the language is to provide an open-ended framework, and let the user decide how to implement something such as the copying of an array.

However, the following does work:

struct myStruct { int num[3]; };
struct myStruct struct1 = {{1,2,3}};
struct myStruct struct2;
struct2 = struct1;

The array num[3] is member-wise assigned from its instance in struct1, into its instance in struct2.

Why is member-wise assignment of arrays supported for structs, but not in general?

edit: Roger Pate's comment in the thread std::string in struct - Copy/assignment issues? seems to point in the general direction of the answer, but I don't know enough to confirm it myself.

edit 2: Many excellent responses. I choose Luther Blissett's because I was mostly wondering about the philosophical or historical rationale behind the behavior, but James McNellis's reference to the related spec documentation was useful as well.