The boxes don't care about shiny paint and interior design. They also are more likely to "pay the freight" than fickle SLF looking for a cheap ride to Las Vegas. "Just in time manufacturing" NEEDS those boxes; the tourists don't "need" the ride...
It's also a form of recycling, just like the used car market. Airlines sell their old airplanes to get new ones. People who can't afford new airplanes buy old ones at a bargain price...