This question is often referred to as the "Fermi paradox of time travel." There are several possible explanations that have been proposed to address this question:
Time travel is not possible: One possibility is that time travel simply does not exist or is not feasible within the laws of physics. If time travel is impossible, then there would be no visitors from the future.
Time travel is limited to certain conditions or constrained: It is conceivable that time travel, if it exists, is only possible under specific circumstances or within certain limitations. For example, it could be that time travel is only possible to the future and not to the past, or that it is restricted to a particular region of space-time. If these limitations exist, it would explain why we haven't encountered visitors from the future.
Time travelers are prohibited from interfering or revealing themselves: Another possibility is that if time travel exists, there could be strict rules or ethical guidelines that prevent time travelers from interacting with the past or revealing their presence. Time travelers may be instructed to observe silently without interfering in historical events or altering the timeline.
Time travelers are simply not interested in our time period: It is conceivable that if time travel is possible, future civilizations or individuals may have little interest in visiting our particular time period. They may be focused on different eras, regions, or significant historical events that are of greater importance to them.
Visitors from the future are present but undetected: It's also possible that time travelers could be among us, but due to their advanced technology or ability to blend in, we are unable to recognize them as visitors from the future.
It's important to note that time travel is purely speculative at this point, and we do not have concrete evidence or knowledge of its existence. Therefore, these explanations are based on hypothetical scenarios and the assumptions one might make when contemplating the question.