Assuming that they have served their entire sentence, they end up in very rual counties where they are MUCH more difficult to monitor, if that's the desire. If they are still on parole or probation you end up with a situation where they are unable to get housing anywhere so they end up living on the streets, where, you guessed it, they are much more difficult to monitor!
I forgot to mention that parolees and probationers are normally forbidden from leaving the county that they were paroled to, or are on probation in. Just leaving is a parole or probation violation.