Why weakref doesn't support built-in types in Python?

Most of the built-in types are not directly weak referenceable (e.g. str, int, float, list, dict, None), and there are a few that cannot even be made so by sub-classing (e.g. tuples in CPython).

Some details about the underlying implementation of weakrefs for several built-in types can be found in this March-2005 python-list post by Raymond Hettinger.