I read about a few sentencings where the judge had sentenced a criminal to a ridiculously long sentence. I read a news article a few years ago in which a judge said that this criminal would be sentenced to a thousand years. What's the point of doing this? Why can't they just say, you are sentenced eternally, without any age limit? No human being has ever lived beyond a hundred twenty, I believe.
Ridiculously long prison sentences don't come about arbitrarily -- they come about because the defendant was convicted of a huge number of crimes, and each instance of the crime is punishable by time in prison. If you murder 10 people, and murder gets you at least 10 years, then you've earned 100 years.
Frankly, there's no good reason to turn that into "forever", and a couple of good reasons not to.
Firstly, although an ridiculously big number might look like forever, it's not. "Eternally" is a lot longer than a century. If you become the first person to survive your 100 year sentence, then you have paid your debt to society and should be freed. A sentence of "forever" would not allow for that.
Secondly, symbolism matters. Those big numbers get attention. Someone gets a thousand years in prison, you know they did a lot of crime, and the government takes that stuff seriously.