He says this to keep the Princess calm and moving forward -- which is their best chance of surviving. He is concerned she might do something irrational like go back or take off in some random direction. He knows that just maybe they will be left alone, and his ploy will succeed. If not, then he has no control over it anyway, and will have to deal with it at that time.
He has also grown into his role as the Dread Pirate. He pulls that role off because it suits him naturally. What would the pirate do? He would down play it and essentially write if off as something to not be concerned about.
Also, it makes for good comedy. He says they don't exist, and then he is wrestling with one of them. I remember heartily laughing at this one.