Because I like it when life has adventure!
Because of the folklore. Reports of wildmen in North America stretch back to Native American folklore. Sa'be is an Ojibwe legend; the Si-Teh-Cah are Paiute; the Tuscarora have a long oral tradition of Giants (whom Christians identified as the moundbuilding culture) and their battles against the Giant Quisquis (probably mammoths.) This isn't even getting into how sacred Yeti is in Tibetan religion. There are no known apes in North America and it has never been confirmed that any Homo species other than Sapiens came to the continent. So what is the source of these legends?
Because Bigfoot is plausible. On the most basic level: upright hominids survived in northern Asia until at least 30,000 years ago. The exact migration of humans has yet to be pinned down; the Clovis Man is now debated on whether it's a direct ancestor of the Native Americans, but 15,000 BP is usually a number thrown around. Ape skeletons are also notorious for not preserving well: raccoons and other scavengers like to crack open our bones and make sure there's nothing to fossilize. If upright hominids existed in Northern Asia when Alaska would have been connected to Asia, then it is very likely that they along with Mammoths, tigers, horses, wolves, and humans would have crossed the land bridge and at least made it to northern Canada.
- Because the Patterson-Gimlin film has never been disproven. It is impossible to replicate the gait that the creature performs in the footage. Muscles can be seen visible under the skin. Fake fur was not yet invented by 1968. If Patterson and Gimlin were so good at faking the creature to the point of designing female breasts for a "costume," then why waste a technological marvel on a 30 second clip of shaky footage? They have a muscle system, an inhuman walk cycle, and patterned fur with no visible seams. Either this is the greatest animatronic of the 1960s, or it's something real.
These are just some of the reasons I believe in Bigfoot. What do you think?
