They're retired or a kid. It's the summer time so many kids have endless hours.
Some people manage to pull it off, but it requires something like 8-10 hours a day just to make one move in each game... so it's common for people to get tired of this within a few weeks, and time out a large portion of their games... so trying to drag the game out many moves sounds like a good strategy.
I am currently playing a player in a Tourney and this guy has over 1300 games going.
Most of his games are 1 day games.

How do they do it?
Is it healthy?
Best of luck to him but I doubt he will be able to keep up. I am playing him very reserved and trying to just get a positional advantage on him. Sooner or later, because he does not have time to think through his moves, he will make that fatal mistake and then I will make him pay.