Why are people so unreliable, even for the smallest requests?

I'm filling out a job application that requires supervisor and coworker references. I've been trying every method possible to locate my supervisors and coworkers from a job I had more than 7 years ago. I've emailed them repeatedly, emailed others who might be able to get in touch with them, called their companies multiple times trying to locate them, and even emailed them on facebook... no replies. One of them even updated his profile picture since I emailed him, but didn't bother to reply to my email. Why are people so unreliable?