They originate that many per year. So that's $5.5B per year for 20+ years. At any given time, they should have income for 20+ years worth of originations, right? Obviously, this situation is more complicated since they couldn't pay.
They lent out much less in previous years, climbing from ~$15B in 2000 to a peak of around ~115B by 2011, and declining steadily since. This makes sense because private lenders controlled a healthy portion of the market before 2011.
The US government doesn't have those billions lying around with no ideas about how to make money with it. It has to borrow it. Interest rates for US bonds aren't high at the moment, but they aren't zero, either.
And as others mentioned, these are the bad loans: those that people do not have and likely never will have the money to pay back or pay interest on.
Because of that, this is more a decision to do the government's accounting differently than a decision to spend money.
This forgiveness costs 7.7 billion so I think they probably would notice.