I sleep a lot, so why am I exhausted?

Insufficient Sleep. Sleep quantity and quality are the main factors that effect feeling fatigued. Different sleep disorders exist that contribute to a lack of quality sleep and appropriate quantity. Circadian rhythm disorders and insomnia effect quantity while obstructive sleep apnea effect both for example. A sleep specialist would need to evaluate you and determine the best coarse of action.
Fragmented sleep. Excessive daytime sleepiness is a sign of fragmented night sleep or a primary sleep disorder (sleep apnea, narcolepsy...). A sleep consult and sleep study can help to clarify the situation.
Many possible causes. Lack of sleep is only one of many causes of exhaustion. As mentioned, you may have a condition like sleep apnea resulting in poor quality sleep, but you may also be exhausted due to anemia, hypothyroidism, chronic infections such as lyme disease or viral infections, intestinal parasites, heavy metal toxicity, mold toxicity, corticoadrenal insufficiency, depression etc. Please see a good doctor!
Quality vs quantity. The quality over your sleep as important as quantity. E.G in osa the patient has poor quality and is there foe is not fully recharged.