Maybe it's a part of human hypocrisy: we want to punish people, but in a way that doesn't make our mirror neurons feel their pain. We want people to be punished, without thinking about ourselves as the kind of people who want to harm others. We want to make it as impersonal as possible.

So we invent punishments that don't feel like we are doing something horrible, and yet are bad enough that we would want to avoid them. Being locked behind bars for 20 years is horrible, but there is no speficic moment that would make an external observer scream.