Mortgage rates have been historically low for several years, but a surprising number of borrowers are still not taking advantage even though rates fell again at the start of this year. How many? Close to 7 million. After the Federal Reserve raised its target interest rate in early December, the common expectation was that mortgage rates would rise. Refinances had already dropped by nearly a third throughout 2015, as rates inched up in anticipation of the Fed's move. "Global economic shocks then sent investors looking for the safety of U.S. Treasurys, driving down yields on benchmark 10-year bonds. Mortgage interest...