It's because attention dilution stymies everything. A new chat window in the web app is the smartest the model is ever going to be. Everything you prompt into its context, without sophisticated memory management* makes it dumber. Those big context frameworks are like giving the model a concussion before it does the first task.

*which also pollutes the attention btw; saying "forget about this" doesn't make the model forget about it - it just remembers to forget about it.