why do people with mutual funds and retirement funds invest in US securities?

Answer 1

Funds have listed goals. Some have given percentages of assets going to different levels of risk. Government securities are considered low risk.
Funds are not always able to find enough stocks that they expect to rise in value. If they believe the market is going to correct downwards or if they are uncertain where to invest they may put the money into cash or government securities. They can settle for a smaller but certain profit with the government securities while waiting for other opportunities.

Answer 2

Bonds are really safe investment becuase they are backed by the united states goverment!