It's espacially weird given that D vitamins can be produced easily in the human body as long as you expose the skin to the sun. It's in no way required to be provided from external sources.

A daily 15 minutes of exposure for the face and forarms is enough, and it contributes to recalibrate the mood and circadian rythm as well. It's free, healthy, and simple. Why would anyone recommand a supplement to people that have access to the sun is beyong me.

Omega 3 is harder, but still.