It's about the demoralizing action such a sentence has. With that large of a sentence you will never get parole, you will never be released early, and you will be an example to anyone else that would consider something like that.
Wait until we start sentencing pedophiles to whole body CRISPR mutation of the oxytocin and serotonin gene so they only get 21 years but become incapable of love and happiness forever. Do you think that would be a better punishment?
Yes make it into a game show where they get a choice: jail forever or 21 years with gene therapy.
If they opt for the therapy: select a random few right after the procedure, make sure you have a camera on the pedos face, tell them they are going to jail forever anyway.