Because it's really hard for your body to absorb, unless taken sublingually (or maybe with a fatty meal, given it's fat soluble).

All that matters is your serum level. 800IU may work for you, but you may need several times that. Best to just get it checked. People feel better at different levels, e.g., I seem to feel best around 35-40 ng/mL.

Check out Dr. Rhonda Patrick (@foundmyfitness) on the topic: https://www.youtube.com/watch?v=uc815fQn8iY.