Is time travel possible?

If so, why haven’t we encountered any real time travelers yet?

Mental time travel (the ability to think about the past, present, and future) is something humans can already do. The kind you mean, not so much. But it’s a huge topic in philosophy and physics. Check out my friend, the Stanford Encyclopedia of Philosophy for a run-down on the pro, the con, and the paradoxes. https://plato.stanford.edu/entries/time-travel/