Do you believe in Bigfoot?
I don’t think an animal should be a “believe” thing. It’s not like asking if I believe in God or true love. It’s either a real animal that exists or it isn’t–it’s not a matter of faith.
My guess is that it’s not possible for a large mammal to go undetected in North America as seamlessly as Bigfoot would have to. Hikers have smartphones, hunters have trail cams, biologists have camera traps and aerial wildlife surveys… I really doubt a breeding community of giant primates could fly completely under the radar here.