For the same reason the government had us collecting scrap metal during WWII. It enlists the citizens in the overall enterprise, even if that effort is wasted. We, well some, are given a sense of purpose, a place in the machine.

Note that I've always admitted that explosive liquids exist (as do solids). The focus here has been on liquids, either individually or in combination, that would pass the puffer or swab test, be combined on board, and be destructive enough to take down an aircraft.

Are we still talking about the same thing?