The problem that many Americans forget, when they are talking about government coverup, is that we would not be talking about 1 Federal government.
At a minimum there would be 3 Federal governments involved, more if sasquatch reach into Central and South America or Asia.
You are also talking about a coverup that has lasted decades, without major leaks.
The Mexican cartels control the border like nobody else, imagine the money they would make off of a body or captured specimen, but where are they.
If there was a decades long coverup going on across all of North America. you don't think that China or Russia wouldn't have at least got a whiff of it by now, and they would like nothing better than exposing a secret like this, even if for no other reason than to embarrass us.
A coverup of this size would involve hundreds if not thousands of people over the last 50 years, and they have somehow guarded this secret, better than nuclear weapons.