Some might want to earn more money so they can retire earlier. Others might actually like their jobs. I've seen people work 10-20 hours more regularly without anyone asking them to.

You haven't worked on the line or a traditional factory job then? There are a lot of "practices" to power game your pay by working in such a way to ensure there is Overtime or to optimise your piece rate.

For example in the UK don't post a first class letter on Friday as work is deliberately kept back to ensure there is OT at the weekend.

Because some people like their work, or because they're in an industry that takes a lot of hours to get going, like starting a restaurant.