I don't know - were the Prophets really all-seeing? They weren't familiar with linear time, they seemed dimly aware of Bajor, but other than that they seemed pretty oblivious to what was happening at our simple corporeal level. Hardly omniscient. Seemed like Sisko was simply bringing the problem of the Dominion fleet to their attention.
Paraphrasing, they said there is no difference between what was, what is and what will come. That would indicate all seeing.
They might be capable of seeing all, but only if they're looking. I don't think they consider our corporeal plane something worth paying attention to continuously. If they are all-seeing, then they can see across billions or trillions of years. Getting them to pay attention to what you
want them to address has to take some effort.