If a fish could remain in the same place forever, then maybe it could figure everything out. Maybe it could live without any mystery in life. Maybe life wouldn't involve elements of confusion. It's probably easier for fish to just remain incapable of thinking about life. As members of the human species, we can't avoid thinking about our lives.
Many explanations exist.
Human history is filled with explorations into the mysteries of who, what, and where we are. While many things have been discovered, countless mysteries remain. The mysteries of existence are present in all of our lives. Mysteries and explanations are interpreted differently by each individual mind. If there is no mystery in life, we can begin living perfectly according to a calculated plan.