But when would you want to have a descriptor as an instance attribute? Descriptors must be in the class dictionary to work:

https://docs.python.org/3/reference/datamodel.html#implementing-descriptors

I suppose, you could want some container class of descriptor objects, but that seems like an extremely narrow use-case, compared to the normal and common use-case of descriptors acting like descriptors. I think special-casing descriptors make sense because they act in a special way.