Why? For tax purposes the definition of a "church" is pretty straight-forward. Congregation, church teaching, spiritual leadership, etc.
A separate non-profit usually won't meet the definition of a "church".
Saying that they aren't is just stating the obvious.
But how "separate" is Catholic Charities from the Catholic Church?
Letter from the Archbishop — Catholic Charities collection will be held at Christmas Masses
How is that different than taking up a Christmas collection for the DNC, another non-profit?