What reasons can there be for a consistent lack of energy?

Lack of energy. Lack of energy is also known as fatigue. This can be cancer-related or non malignant. Cancer related fatigue is an extreme feeling of lack of energy to minimal effort. Common causes may include uncontrolled pain, depression and thyroid problems. Even in cancer patients, non malignant causes abound, such as, shoulder joint pain from rotator cuff tendinitis, sleep apnea and anxiety/depression.
Many causes. Fatigue can be related to a real medical condition such as infections, inflammatory conditions, cancers, heart disease, lung disease, and mental health. If your doctor has checked for this other persistent causes can be lower levels of vitamins, hormones, minerals, poor diet, poor sleep and in essence, an unhealthy lifestyle.