U.S. airlines are saving tens of millions of dollars every week because of lower prices for jet fuel, their largest expense. So why don't they share some of the savings with passengers? Simply put: Airlines have no compelling reason to offer any breaks. Planes are full. Investors want a payout. And new planes are on order. In fact, fares are going higher. And those bag fees that airlines instituted in 2008 when fuel prices spiked aren’t going away either. …