Companies are always eager to improve their bottom line. Human resource department is consider a 'cost center' instead of profit center. HR staff live in fear of being laidoff when it come times of cost cutting compared to their colleagues in profit generating roles such as sales.
1 thing that's myopic that HR departments do when it comes to recruitment is: Only shortlisted candidates will be notified.
The rest of the candidates are left waiting & wondering if they are selected. Rare is the company replying to them that they are not selected. Hence they are missing out an opportunity on creating goodwill. These candidates not selected wouldn't have applied to work in a certain company if they don't consider good or at least neutral about working there.
By replying to these unselected candidates, there's a chance of them becoming future customers or even more loyalty. Is a cheap way of advertising/marketing.
Just because the rest of the herd in the industry can't be bothered to waste their time communicating with these unselected candidates doesn't mean the 'conventional' way of the herd is appropriate for the situation.