Or it might be because they actually do something that benefits the authorities. Sometimes, the prisoner may actually have all the resources necessary to escape, staying "imprisoned" because there's a particular reason to do so, or because they can run their affairs perfectly unhindered from right where they are. The character in question has the wherewithal to bribe the guards, walk freely through the prison, have their plumbing and bedsheets upgraded, eat caviar in their cell instead of baked beans in the lunchroom, etc. A big-time crook gets sent away to prison, but the forces of law and order can't sever all their outside connections.