Hate to play devil's advocate here, but shaming nonresponsive would-be employers has been tried and has failed many times before.
If an employer has time to run an ad, that means they're short-staffed and pressed for time. Sending heartfelt thank-you's, even at five seconds apiece, will not fix problems on the employer's end. It is not their job to make rejected applicants feel better.
Employee money and employee time are worth the same, by definition.
I agree the system is majorly due for disruption, and as a proud occupant of the tm;dr circular file for many of the targeted resumes and cover letters I've sent out, it does get extremely frustrating. I'd love to get an enthusiastic job offer from those who have snubbed me and treat them the same way, but that's just not how it would work for most people. Workers are plentiful and positions are scarce, even if salaries are high.
The parity in the system is maintained with the tendency for these lazy offerors to dissuade the best applicants from applying for their jobs. (Isn't it funny that the snubs that are the absolute worst are the ones that took you an hour of dedicated busywork to apply for, even though logic would dictate that a company with a more reasonable application process would average out to be a better job?)
If you're young and bright enough to be able to do that job; disrupting the hiring system for the better, rather than complaining about how unfair the world is, can't be any less worthwhile, even if it has long odds. The world will beat a path to your door and consider you a hero if you can find a way to fix this broken system.
Despite the heartache, most job seekers could care less about getting a dear john, especially weeks or months following.
HR's problems as an industry run far deeper. Employers spending too much time and money for too little long-term result; applicants spending too much time (and money!) for too little long-term result. Find a way to fix that wicket and this other little problem becomes that much less important.