What are the odds of some Soldier or Marine or Border Patrol or DEA agent, etc., seeing one? Probably very high. But even reported, it likely stops at the first level chain-of-command because its not relevant to each agency's specific statutory missions (national defense, border security, or stopping illegal drug activity) and, as noted in another thread, even if reported to the correct agency (National Fish and Wildlife?), still not sufficient to establish that a species exists. Even if the witness was the offspring of George Washington and Mother Theresa and genetically could not tell a lie, and was hit with truth serum, and passed multiple polygraph exams, its just not enough.
Sadly, unless and until John McClane tosses the dead body of a Bigfoot out of a window at Nakatomi Plaza onto a police car below, efforts to prove existence will be futile.