Good point. A lot of that evidence was shipped to China as scrap metal. By now a lot of that has probably already been recycled back into USA by way of cheap imported merchandise.
While they are at it, they should consider another question that derives from the first.
How did 200,000 tons of steel disintegrate and drop in 11 seconds such that a few days later, while this massive pile of rubble was still burning, HSA / FEMA, by way of one of ther crack operatives, was able to pluck from said pile a passport, in near pristine condition, allegedly belonging to one of the hijackers?