The numerous people who aren't only in those positions for the pay. Some people thrive on the stress and challenge of certain jobs. They want to do those jobs. This is evidenced by those who go on working long after they have any financial need.
The need for personal fulfilment won't die without financial incentive. If you have the capacity to do a job which stimulates you, it's highly unlikely you will select a job that bores you or that you find actively distasteful.
There is an argument that removing the financial equation of employment rather frees people up to do what they are suited to, rather than what will get them x y z salary. Because we don't live in that sort of economic set-up, we naturally imagine 'everybody' will only ever want to take the 'easy' option. But is the 'easy' option really doing something you have no interest in? Wouldn't that make it stressful, even if the job itself isn't stressful?
Added to that, jobs have different stresses. It may be more mentally taxing to be a CEO, but that doesn't mean all the CEOs would jump at the opportunity to be out in the rain digging a ditch if it paid the same, because that's going to be taxing in a different way.
Not everyone is just looking for a cushy number, and even if they were, most of the NMW jobs people perceived as 'easier' from the point of view of stress actually aren't. The stress is just different.