This is because the fuel is carefully manufactured so that it cannot explode in fight. If there was even the slightest chance that a 747 could exploded spontaneously, do you think the Air Force would still be using a 747 as Air Force One?
Of course, one person that I know pointed out that Air Force One and TWA 800 were both of the same 747-200 series. He was questioned by the Secret Service because of this factual comment.
Maybe they just wanted to give him more accurate information. To start with, TWA 800 was a 747-100. Not a 200. And it was built almost 20 years before the 747's used to transport the President. Air Force one is such a highly modified 747-200 it is essentially a unique airframe.