Why has inflation in the U.S. remained so low, despite such low interest rates for the past 15 years?

Low inflation is what allows low interest rates. High inflation causes money in banking accounts to be worth less, over time.

Therefore, to get people to deposit money into bank accounts, banks are forced to offer higher interest on the balance. In turn, the higher interest the banks pays, forces them to charge higher interest on loans.

Because the US is currently broke. It's public has no money, only the 1 % billionaire's do.