They gain the goodwill of a prospective employee who might cycle back and be a great fit later on in their career (and who has more skills then). It's also about what they don't lose, which is not only me but potentially other prospective candidates. Know what happens when someone mentions they are considering applying to a place that never got back to me? I tell them don't apply there, they waste your time / they don't care about candidates / their interview process sucks. How many people does that person then go tell? I've absolutely punted on applying to places which my friends have said similar things about.
Reputation matters a lot.