What causes a person to be extremely tired all of the time no matter how much sleep you get?

Sleep problem. Very common in people with any primary sleep problem. Specifically very common is chronic insufficent sleep and sleep apnea.
Different reasons. You need to have your fatigue worked up. There are tons of reasons to be tired. Common ones are anemia, thyroid dysfunction, sleep apnea. I usually obtain good history, do physical examination and order some bloodwork like complete blood count, thyroid function test, comprehensive metabolic profile and if suspicious for sleep apnea, refer people for sleep study.