Traveling Back in Time: If You Could Go Back Nine Times

If a time machine really exists, why haven't we seen a person from the future with conclusive evidence? It may be that humans in the future know that going back to the past may cause irreversible chaos to history, so they choose to stop before going back to the past. It is also possible that every year we go back in time will consume a huge amount of energy, and our present has become a distant shore from the perspective of the future. Perhaps at the last moment of the demise of human civilization, after exhausting all human wisdom and hope, no one built this magical machine that can go back to the past. But when a time machine really appears and gives you nine chances, can you really change the world?