Boneyards aren't cemetaries. They only keep airframes that are currently in service, that may or may not be in production anymore. That way any engine replacements, wings, or sundry parts, can be scavenged to keep the aging fleet still in service. As soon as that plane type is no longer in the inventory, that is to say as soon as you wont need those parts anymore, they will start scrapping or selling them to make room for the next planes in the inventory.
I think the youtube clip said it was taken between 1946-1948, so B24s, B17s, and B29s would still be in service at that time.