The fact we have no evidence of anyone coming back in time from our future strongly suggests it isn't possible
I'm not sure that's the case.
Taking a huge leap into the improbable...............If time travel were possible using weird science as yet unknown to us, it would not be something that would be able to be done easily. So if the technology was developed the governance around it would huge and it would take, I imagine, vast resources to accomplish it. Probably vast amounts of energy too. So it would be unlikely to be done by maverick factions and 'day-trippers'.
Think how
we now consider that making contact with any kind of undisturbed life (eg bacteria under the ice sheets or possibly on Mars even). The scientists think very carefully about avoiding damage to what is pristine and do their best not to affect/ destroy what is there.
If aliens, or even us from the future were among us would they not have similar moral constraints?
I read an article by a prominent scientist a who posed the question and even put forward as an argument for the lack of 'visible alines'. That 'why would they bother with a small planet with only relatively basic technology'.
I could hardly believe my ears. For God's sake our naturalists get in an excited panic if a new slime mould is discovered let alone a sentient technological species.
Edit: Just because you can't see something doesn't mean it's not there. A bit like fleas on a cat.