People think it's just being put somewhere by yourself. It isn't.
You are put in a room, often with no furniture or even heating, a room not much bigger than you have room to lie down. If you're lucky, they give you a blanket, but you might not even be allowed to wear clothing. The lights never turn off. There is no stimulation at all because you're not allowed even a book to read. You hear endless screams from other prisoners in solitary shouting through the vents either because they're mentally ill or they're desperate for some sort of human contact. You might be left there for months.
People who aren't mentally ill when they go in often are when they come out. If they come out alive. And many of the ones who do come out alive take their own life due to what happened to them.
It is simply unconstitutional. It is very hard to argue that isn't cruel.
Unfortunately, many Americans think they deserve it for being criminals.