I've been in the situation of working 10 hours/week. I'm pretty sure that it's as damaging to your mental health as overwork. It's like being obese because you don't exercise and not exercising because you're not fit enough to.
I often define work-life balance as a point where you have enough of a life to escape from work and enough work to escape from life. When there's not enough work, you don't actually get that escape.
Obesity has much more to do with diet and genetics than exercise. The human body is freakishly efficient and you’d need to run for miles to “exercise out” a couple of cookies.
Why? Maybe they've already worked hard and made enough. Or maybe they're like me and worked hard but was never rewarded for it, so why continue trying.
The most charitable interpretation I can give this question is to treat it like a hypothetical. From that perspective, this is quite a fun question to ask!
Even more difficult when you add some mental health problems on top of it.