Because while use of photoshop is widely known, many are still unaware that video can be edited like this and look so real.
I do believe that some people will fall for them, but the damage won't be any worse than when people fall for shopped images and phishing emails. It's just something we have to deal with.
I mean they can accept just about any kind of special effect in a movie as being possible, but editing videos to swap faces is a huge stretch?