You're ignoring the fact that a waiting human costs money which is actually the primary reason why AI hasn't resulted in significant productivity increases, while overly focusing on the human being the bottleneck. The point isn't to monitor the session, the point is to verify the output with human inspection. If the agent is fast enough, attending to the agent can make financial sense.

Human memory has a finite capacity and decays over time. If you do many inspection tasks in parallel you are splitting that finite memory. If you do the same tasks sequentially, the human can give instant feedback to the model without costly context switching. This in turn increases iteration speed and reduces total labor costs. This is no different from developers preferring JavaScript or Python for immediate feedback and fast iterations.