The article says this one was on a pylon (with a spare engine on display on the ground below). Being owned by NASA is important, also being on a military base (the home of USAF test flight!) where it could be removed and easily wheeled into a large-enough hangar without anyone noticing for 5+ months.

Getting any of those planes outside of their museums would be logistically difficult, and of course it would immediately be noticed. And even if you can rent a hangar big enough, and secure it, then you're stuck at BFI/IAD/Wright-Patterson trying to do this impossible task.