The simple answer is: that’s just the way it is, I’m afraid.
Likewise List<T> doesn’t override GetHashCode or Equals. Note that it would have very little way of formatting pleasantly other than to call the simple ToString itself, perhaps comma-separating the values.
You could always write your own extension method to perform appropriate formatting if you want, or use the newer overloads of string.Join which make it pretty simple:
string text = string.Join(",", list);