There are responses to the OP's question, including skeptoid armchair psychology (our need for thrills and quests and the like.) Then there's the basic flat out chutzpah statements that anyone "claiming" to see a Bigfoot is obviously a liar, because they need to feel like they've put something over on the rest of us. As JREF member "Deacondark" posted:
... Claiming to have seen one accomplishes two goals: You get the wonderful feeling of "getting one over" on people, and if it happens to be highly educated, intellectual types, all the better. And: You become part of an "elite club". How many times have you heard a footer say "you can never understand until you see one yourself" They love the feeling that they have seen something few have witnessed. I believe this is enough impetus to cause them to believe they have seen something that they really have not.