In the past, these people were usually in prison for a long stay. But we just had a person go threaten the life of a judge, leave to go commit two armed robberies, after just getting out of prison after serving one year of a five year sentence for double attempted murder.
We just don't put monsters in prison nearly as much as we should.
Since peaking in 2009, the US prison population has dropped by 2.3% per year, with a 14% drop during COVID. This has gotten a lot of innocent people murdered as there is no theory of justice that can rationalize only giving someone a single year in prison for attempted double murder, especially when that person gets out and immediately goes on to threaten a judge and then later that night does a double armed robbery.
As El Salvador shows, yes, you can incarcerate your way out of endemic violent crime.
We just don't put monsters in prison nearly as much as we should.