That's the point. A person doesn't have to be a working royal in order to do charity work. The work they do isn't less meaningful because it doesn't get mentioned in the court circular.
The fact that there's a long list of charities which do not have royals as patrons is not a reason to add more working royals or a reason to expect Beatrice and Eugenie to quit their jobs and become dependent upon Charles to pay for their living expenses. And if any of those charities wanted Beatrice or Eugenie to represent them, they would have asked them to.
And I don't understand how having Beatrice and Eugenie take on vacant patronages helps lighten Charles's, William's or Catherine's workload. It's not as if either woman could meet with representatives of the Duchy of Cornwall, the Earthshot Prize, Catherine's Early Childhood Years, or whatever Charles's Prince's Trust is now called.
206