Why is there so much confusion between the terms Ergonomics, Biomechanics, Human Factors, User Interface Design, and Human-Computer Interface?

Because all are studies related to improve product usability and its USER EXPERIENCE. If the product is a hardware, talking about a computer mouse, a chair, a massage tool handle... We address improving its usability by studying their ergonomic, human factors or biomechanics. I.e. how they will easily be handled by a human being in accordance to his physical body dimensions and movements. If the product is a software, an application, an interface of a computer/screen based product we talk about human-computer interaction, interface design