I am inclined to think that all politicians should have an enforcable upper age limit, but as there isn't one, I shall be charitable here.
Perhaps President Biden does not feel he can in decency leave the ship of state to the tender mercies of Mr. Trump, whose supporters could be stupid enough to vote him back into power.
President Biden could also rightly feel that wit war in the Ukraine, civil war in Sudan, the Middle East in its usual turmoil, as is more than Sudan in Africa, and China casting lustful eyes om Taiwan that now is not the right time to stand down.
If he becomes President again, what is the worst that can happen? The vice-president having to step in if Biden becomes seriously ill or dies while in office, might be a very good thing.