Why can't we reach agreement by updating on the beliefs of others, as per Aumann's theorem? Why do we need to describe how we arrived at our beliefs, instead of simply stating them? In fact in ordinary conversation no one even tries to use Aumann-style negotiation, and from my perspective this seems completely rational. Even when talking to people whose rationality I trust, assuming common knowledge of rationality or honesty is never close to accurate; if we abandon skepticism we can expect to be consistently wrong (even when we aren't deliberately manipulated).

This is an interesting problem. I agree that Aumann-style updating in practice will lead to less correct beliefs systematically, but you don't seem to analyse why it is so. My hunch is that the reason is our inability to judge other people's rationality well, but I'd like to know what do you think about it. The explanation

we aren't sufficiently perfect rationalists to use Aumann agreement

doesn't tell much.