By analogy: no agent can summarize War and Peace, but several agents can, Peace-wise (sorry). Like AI map reduce. The question is thus "why not use one agent for this recursive merger?" Answers maybe being:
1. Different scholars (Russian lit. agents, ...war strategists?, etc) pay attention to different things with valuable insights
2. Multiple readers parallelize well, and some are faster than others
3. Managers can direct talent to (re)read chapters most relevant to their specialties, and coordinate meta-learning and communication
You might not get much mileage out of this approach with book summaries, but other domains are a different story (sorry).