> In any other domain, I would agree with you, but charities are at least theoretically about maximizing total gain to society,
No, they aren't. That is government.
Charities are organized for a specific identified charitable purpose and are generally not, even in theory, any less narrowly focussed on that identified purpose than a for-profit firm is on serving the common, usually financial, interests of it's stockholders.
> This is why competent nonprofits are judged by metrics like administrative efficiency nowadays.
The administrative efficiency measures used by outlets like Charity Navigator are feel-good metrics that favor unevaluated, uncontrolled funnelling of money in the hope of achieving the notional objective. A real charity efficiency measure would measure its cost-effectiveness at achieving outcomes that are its charitable purpose, not administrative vs. program funding measures (and, as a consequence, would probably only be directly comparable within the same domain.) Administrative efficiency is used because it's cheap, easy, and portable across domains, not because it's even remotely meaningful.