Why isn't == used to compare structs

[Jacob Carlborg]

struct String
{
	char[] data;
}

void main ()
{
	auto foo = String("foo");
	auto bar = String("foo".dup);

	assert(bar == foo);
}

Why isn't == used to compare the struct members in the code above? I 
mean, if I compare the structs with == it could also use == to compare 
the members. If I use "is" to compare the structs it could use "is" to 
compare them members.