In my opinion, the reason for it is that not many people know how to prepare for an interview and gain self-confidence.

self-confidence 是由前缀 self-（表示"自我"）和名词 confidence（信心）组成的复合名词。self- 强调与自身相关，confidence 指"信心、信任"。合起来表示"对自己的能力和价值的信任感"，即"自信心"。