Getting the relative most out of the bottom line workers and finding and taking care of the underperformers is the direct responsibility of their managers, and I'm saying this unironically. Managing is about being a performance multiplier, and the most hands-on part of management always involves concrete teams of people doing concrete dirty work.

It's the manager managers that are the problem mentioned here.

The incentive structure is that those workers capture much less than the value of their total output. If they worked harder so the firm required less employees, how do they benefit?