It's true, I checked the recidivism rate, and it's surprisingly low. But that is mostly because they're identified to the public. A thief can change their financial situation and stop stealing. But a sexual predator's urge usually remains.
I went to a seminar a number of years ago - this was was primarily on paeophiles. The researcher (I think from the U of t) was somwehat sympathetic to paedophiles - saying there are many different kinds. Some are absolutely predators and will continue to re-offend. Some are more offenders of opportunity - don't leave your kid alone with them, but otherwise aren't going to try and hunt down children,
And there's a whole population of people sexually attracted to children who will almost certainly never harm a child - because they know it's wrong.
It's the same with sexual offenders. Not everyone who commits a rape is necessarily a sexual offender. They may well have done a terrible thing just because they were in a particularly bad situation, with an easy victim.