If you're a grown adult, and the job in question takes up your working day, then you're damn right that it should remunerated well enough to be enough to live on.
No, because not every activity a grown adult can do is worth $livable_wage. Say some guy likes to repair VCRs... they're a dying breed, but he likes working on them. The market has decided that VCRs aren't valuable any more. Likewise, a person with a broken VCR is unlikely to value repairing one very highly. But if some guy wants to do it for a price the market will pay, so be it.
Or take volunteers. Why is it ok for someone to work for $0 as a volunteer for a full day, but paying them $5/hr isn't ok?