There is another reason to not respond to an unsuccessful applicant: when they try to turn your "no" into a "yes." And then there were the truly crazy, who wanted to take personal offense at not being chosen.
I had a work camping job after I retired which involved hiring. At first, I tried to respond to each applicant, at least to say they were not a good fit. After getting blasted and connecting with professional time wasters, I gave that up as a bad idea. Most people were nice and appreciated a response, but even though only 10% were crazy, that was enough to keep me from blanket replies in the future.
