Why has social engineering become so attractive to the adversary

Prepare an 8-10 PowerPoint slide presentation that could be presented during a lunch knowledge transfer session workshop. Building on our discussion topic, why has social engineering become so attractive to the adversary?
We have firewalls, anti-virus, anti-malware, and all of our systems are patched, why do these attacks continue to work so well?
What is the psychology behind a social engineering attack?
What type of research is necessary to perform such attacks?