Saying “I dont know” is the most reasonable approach. We dont have any evidence, therefore we dont have any reason to believe something happened,at least at the moment. Until there is evidence, then we can begin to investigate, and then we can see where the cards fall.
In the meantime, we can certainly use our reasoning to deem what is and isnt probable. In all these years there hasnt been a single corpse, not one remain, not one legitimate DNA sample, not one public encounter of a Bigfoot, therefore its reasonable to conclude that he most likely does not exist. To say he definitely does exist in the face of all that abscence of evidence, would be unreasonable.
But no believer says “I dont know.” They always seem to know with certainty that Bible events happened, that God exists, etc. without a single shred of evidence.