how could big foot live for thousands of years and where is all its offspring?

Answer 1

Presumably the same place as his waste products, bones of animals he's eaten, old campfires etc. Perhaps these things just can't be seen unless you have an IQ of 60 or less.
Perhaps he's like the Phantom, when he dies his son takes over, everyone thinks it is the same being.
Bigfoot - the Hoax that cannot die.

Answer 2

There are no such creatures nowadays, however there were giant hairy apes called Giganthropithecus that were up to 7-8 feet tall that had surive in Asia for 10 million years only recently to become extinct 250,000 years ago. Perhaps that the natives had seen the giganthropithecus and pass on the tale through generations long after the giganto became extinct. That would explain why the natives tells story of bigfoot or giganthropithecus.
There are no more bigfoot, they were real only to become extinct 250,000 years ago.

Answer 3

That's a good point. Unless he's alone and immortal, you'd think he'd have a family!