Most employers are willing to soak up the waste, in order to have access to their staff throughout the work day. And most employees are eager to get the extra pay, even if they aren't spending the extra hours productively.
As an entrepreneur, I would love a 20 hour a week job that paid $40K a year -- it would provide financial stability and free time, allowing me to bootstrap businesses indefinitely. Most people aren't looking for that type of position, however. They're looking to maximize take-home pay.