But they can and do. Previously:

What if I told you that our federal taxes don’t go toward food stamps at all, and in fact pay for nothing?

• The United States can always pay any debt it has because we can always print money to do that.
  — Former Fed. Res. Chair. Alan Greenspan
• Why The Government Has Infinite Money
• What Caused Hyperinflation In Weimar, Zimbabwe And Venezuela?