Actually, most don't, and aren't certain at all. They know quite well what the limits of their theories are and if someone is able to follow their thoughts are usually quite willing to explain. I know of no other group that says they are so uncertain. You want certainty listen to politicians, programmers, and clergymen.
Can't agree with the former, although I would say that the latter would be true if they were challenged on it. I can't count the number of PhD's that have mouthed the equivilent of, "the astroid strike caused the mass extinction of..."
Its safe to say that scientists never think that their theory is wrong (why would they hold it otherwise), and it is human nature to just assume what you must in order to procede on to the next step. The problem with not acknowledging the level of uncertainty arrises when we are told we must act on the science and we can't question it because there isn't sufficient proof to the contrary, nevermind that there is very little supporting proof. e.g. I've blindly pulled one black marble from a bag of unkown size and no other marbles can currently be extracted, THEREFORE, all available evidence is strong that all the marbles in the bag are black.