Well we know he's only a neet by choice, he has worked and travelled around the world in the past, and is not particularly lazy while being extremely intelligent, resourceful, brave and outgoing. Even one of his former classmates was surprised he was a neet, so why he's still a neet? Well, one possible reason would be because of his sister. She's timid by nature but probably something or another happened during the time he travelled around the world and he chose to be a neet alongside her so she doesn't feel left behind anymore.
Now until we get some more backstory for Haruko it's conjecture, but who knows when we'll get more of her backstory.