Why is it often hard for an EMT to determine which musculoskeletal injury has occured?

it's difficult sometimes because there are,so many different musculoskeletal injuries such as,

Tendonitis. This is an inflammation of a tendon – the fibrous tissues that connect a muscle to a bone,

Osteoarthritis,

Rheumatoid Arthritis,

Bone Fractures,

Carpal Tunnel Syndrome and

Fibromyalgia.

answer; the above is true;