How do you know they're holding them down?
What would/should they be? How do you know?
That’s the whole reason for the Fed’s quantitative easing; injecting trillions of dollars into the economy and intentionally lowering interest rates in hopes of stimulating the economy. Read up on QE and you’ll understand.
Now the national debt is so massive that allowing interest rates to rise to normal levels will increase the debt service cost to the government (i.e., it will cost a lot more to pay the interest on the debt and it will compete with funding needs in other areas of the budget).
The Fed is between a rock and a hard place with few arrows left in its quiver.