In other words, when I say "what's the evidence for that?", it's not that I don't trust your rationality (although of course I don't trust your rationality either), but I just can't deduce what evidence you must have observed from your probability declaration alone even if you were fully rational.

The argument can even be made more general than that: under many circumstances, it is cheaper for us to discuss the evidence we have than it is for us to try to deduce it from our respective probability estimates.