Probably because with fewer tabs, you actually get to do the deeper thinking that you enjoy? With more, you're constrained to quickly converging on the best way to keep the plate spinning
(That is also an unproven statement, obviously, but that’s what I’d want to study/test)
People who keep dozens of agent plates spinning all the time must have a broken understanding of productivity where they think more code = better regardless if anyone needed the code. In the old days engineers burned out when they got a promotion and thought they now need to write more code to compensate, maybe burnout in the agentic age looks different.