I've been in a lot of state and federal prisons in the US and I've never seen what you describe. The only inmates that are left alone in cells for days at a time are so extraordinarily violent that they can't be let out without someone getting hurt. Even they have interaction with health and security staff, but it's through a window.
Disciplinary segregation in every prison I've been in usually have two people to a cell whenever possible (which results in much lower suicide risk) and they're let out several hours a day if they're safe to do so. They can and do talk to people in adjacent cells all day long.
It's still fucking miserable, but it's not what you think.
In Norway, they get much more out of cell time in segregation as long as they're safe. There are people that are only let out of cell alone, if they're really violent.
In Scotland