California Magazine recently posted an article on their website examining why people believe that Bigfoot could be roaming the forests of the United States.
Reason 1: They think they saw Sasquatch, and they want to prove to themselves and the word that they're not "crazy."
Reason 2: Their tribe believes in Sasquatch, so it would be weird if they didn't.
Reason 3: Believing in Bigfoot keeps hope alive that people can be self-sustaining - and that humans haven't totally destroyed the environment beyond repair.
Reason 4: Lack of proof doesn't disprove that Bigfoot exists, so it's hard to declare - with certainty - that Bigfoot is fake.
Read more into their reasoning on their website.