Why do descriptors (and thus properties) only work on attributes.

[Antoon Pardon]

Can anyone explain why descriptors only work when they are an attribute
to an object or class.  I think a lot of interesting things one can
do with descriptors would be just as interesting if the object stood
on itself instead of being an attribute to an other object.

So what are the reasons for limiting this feature in such a way?

-- 
Antoon Pardon