Why can't we declare a variable length array in struct in C?

Question

It's usual to get the length of an array and then declaring it. Then why can't we do the same in a struct?

it's like...

int arr[n];

is acceptable but

struct number{
int arr[n];
}

is not acceptable.
In both n is taken from the user using scanf before declaring the array. So why does the compiler not accept the second form?