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?