So, when I walk across the US/Mexico border, I somehow received clearance from the US government to do so? If I drive or take a bus across the Canadian border? If I take a speedboat or sailboat or cabin cruiser from Ft. Lauderdale to the Bahamas, I somehow received clearance as well? You guys are assume that air travel is the only way to leave the US. That is not true and is in fact a very small percentage of crossings per day. So, don't confuse regulations for international air travel out of the US with leaving the US in general.

Back to the OP's point, again, if you want to reconfigure every US airport that has a departing international flight, think about those costs for every airport. As someone who travels internationally at least once a month - for close to 15 years now - and have flown, driven, walked, bussed and gone by boat and ferry as well, there are a lot of ways to leave the US that don't involve buying a plane ticket or applying for a passport for each and every instance of international travel.