If I reject someone who has made the interview stage, I always make sure to contact those candidates by phone/mail (for people who made it to the in-person interview) and e-mail (for those who made it to a phone interview).

For simple applicants, however, I always make sure the ad says 'only those qualified for interviews will be contacted'. Applicants should know from that sentence that "no news is bad news", shouldn't they?

When you're hiring these days, you get a gigantic stack of resumes for each posting. The bigger the pile, the more arbitrary the reason a resume or application is trashed.

Not to sound cold, but people who are hiring try to spend the most time on finding and interviewing the best candidates instead of appeasing the ones that didn't meet their requirements.