That is a good question. The answer is, I don't know. Whenever the question is posed, we always come up with an escape clause because thats what we do. We're busy trying to figure out how its possible, not how it isn't. A few of us are operating from the position of knowing that Bigfoot exists, and are merely offering up their best guesses. Most of us are operating from the assumption that Bigfoot exists and are just doing our best to explain it.
However, I've always found that particular question rather unscientific and a bit arrogant. We can travel at hypersonic speeds, we can split an atom, heck we can even put a man on the moon. If our technology is that great, then surely we would have been able to use it to catalogue most, if not all creatures that exist on this Earth. If there are any left, they are either exceptionally small or exceptionally remote. There is no way there could be ever be an 8-foot tall, 800 pound species of ape living in one of the richest and technologically friendly nations in the world! That is just preposterous!!
To clarify my point, the question is unscientific because it is based on an assumption, and that assumption is arrogant because it is based in the egotistical idea that we will soon know all there is to know about the planet on which we live.
Scientifically speaking, we can only state two things.
1.) That the existence of an 8-foot tall and 800 pound species of ape living in North America is currently unconfirmed.
2.) We have been unable to obtain clear and unquestionable video or photographic evidence with this creature as an subject.
Any assumptions as to why, whether they be in the positive(for the creature's existence) or in the negative(against the creature's existence) is merely speculation and is not permissible in a scientific debate because they can neither be confirmed nor denied.
I'm not trying to say that we should put a stop to speculation on the Bigfoot Forums. If we did that, there wouldn't be hardly anything left to talk about. However, let us not make the mistake of thinking that our speculation is an indication of the existence or non-existence of Bigfoot as a whole.