It's an interesting question and one I have asked many times on a variety of subjects. I think Crux nailed it that it's more about what people choose to believe.
A reoccurring theme that I see over and over, whether it's cryptids, ghosts or even current events, is that people confuse the ideas of proof and evidence. Simply put, evidence only informs a belief whereas proof confirms a belief. So regardless of what you believe, anything you have seen or experienced that led you to believe it is evidence. Even if someone else looks at the same evidence and comes to a different conclusion or belief, it doesn't cease to be evidence. It only means that different people have different thresholds for how much evidence it takes to change their mind. In this same vein there are people who are set in their beliefs regardless of not just evidence but actual proof. Often these people will discount any evidence without actually looking at it, usually it's circular logic of the "This evidence must be fake because these things aren't real, if they were real there would be evidence" variety.
When it comes to something like Bigfoot there is a lot of evidence which both sides can point to without compelling each other out of their chosen beliefs. Eye witness testimony and historical records indicate that there is some sort of unclassified creature living in certain forests.
Science tells us that there are certain dietary and mating requirements for such a species to exist and that there should be contemporaneous evidence outside of sightings and foot prints available. These two sets of evidence are at odds with each other and don't seem to have a way to both be proven right, one side has to "win" and the other "lose".
It also doesn't help that there are hoaxers and people who create fake evidence, or when a group of people present evidence then years later some members of the group claim it was a hoax but others still insist its real. Which leads to valid skepticism around other such evidence. On the scientific side one has to make certain assumptions about the creatures physiology then form their hypothesis without knowing if those assumptions are even accurate. This is similar to how many paranormal investigators operate, they assume that ghost have certain traits or behavior and use equipment they think will react with or reveal such things. In both cases an lack of evidence can mean a bad assumption, wrong place, wrong time or all manner of other inconclusive conclusions.
Which is why Bigfoot, and other such phenomenon, continue to go unresolved. Each side has too much evidence to be dismissed but not enough to provide proof.