If knowledge and skill comes from time immersed in work and study, then the knowledge worker who puts in 60 hours a week will be far more valuable than the one who only devotes 15 hours a week.
A surgeon who practices medicine only 2 hours a day will be much less capable than one who does more surgeries, spends more time learning new procedures and technologies, and keeps current with all the latest research.
If you need to design a suspension bridge, do you go with the engineer who spends 80% of her time not designing bridges, or do you choose the engineer who lives and breathes bridge design and has many more successful projects in her portfolio?
An attorney making $600 per hour might survive financially on 4 hours per week if her hourly rate stayed constant. But if she tried to do that, her knowledge would quickly atrophy, and nobody would pay that rate any more.
It's a bit of a paradox. To make enough money per hour to live well on a small number of hours, knowledge workers must acquire a very high level of knowledge and skill to make their time a scarce resource.
But to acquire and maintain that level of knowledge and skill demands a large and continuing investment of time.
Competition with other knowledge workers ensures that the highest rate per hour goes to those who spend the most time actively engaged in their field.