Why is Collection not simply treated as Collection<?>

The major difference between raw type and unbounded wildcard <?> is that the latter is type safe, that is, on a compile level, it checks whether the items in the collection are of the same type. Compiler won't allow you to add string and integer to the collection of wildcard type, but it will allow you to do this:

List raw = new ArrayList();
raw.add("");
raw.add(1);

Actually, in case of unbounded wildcard collections (List<?> wildcard = new ArrayList<String>()), you can't add anything at all to the list but null (from Oracle docs):

Since we don't know what the element type of c stands for, we cannot add objects to it. The add() method takes arguments of type E, the element type of the collection. When the actual type parameter is ?, it stands for some unknown type. Any parameter we pass to add would have to be a subtype of this unknown type. Since we don't know what type that is, we cannot pass anything in. The sole exception is null, which is a member of every type.

The reason is quite simple:

You may read Objects from a Collection<?> the same way as from Collection. But you can't add Objects to a Collection<?> (The compiler forbids this) whereas to a Collection you can.

If after the release of Java 5 the compiler had translated every Collection to Collection<?>, then previously written code would not compile anymore and thus would destroy the backward compatibility.