Because it can make them more mysterious and then more reliable. Let others know that he is not looking with his eyes, but calculating with his heart. Another point is that they dare not look into other people's eyes, because they are guilty, so they poke their eyes.
When many people go to fortune telling, the first thing they see is the appearance of the fortune teller. They think that young people are not allowed to be liars, but those who wear sunglasses and are old are allowed to tell fortune. Because they think wearing sunglasses will be awesome, because they don't look with their eyes, but calculate with their hearts. Just like people in TV dramas now, very powerful people wear masks. This will not only show their mystery, but also show that he is different from others.
Many people rely on mystery to attract people. For example, if you see a sneaky person on the road, you will want to put your eyes on him, but if you are an ordinary passer-by, you won't look at him. This may be some kind of law. People generally think that those mysterious talents are the most reliable. The beginning of some novels will also be written with the idea of arousing readers' curiosity, because this will make readers more curious to read, rather than reading the story with a guess.
They may wear sunglasses because they are guilty. In their line of work, they have to tell people's fortune without conscience. Since they have no conscience, they will definitely feel a little guilty, and then they will poke their eyes blind and wear sunglasses, so that they can't see the fortune teller and the fortune teller can't see them either. It is never possible for anyone to predict the future or guess your fate in the future. These are all tricks, just like magic. They will try their best to describe what happened to you recently, because they want you to trust them more and make you feel that wow, they are all right. In this way, you will trust those fortune tellers more, then listen to them, then take notes, and then you may come to this person for fortune telling.
Fortune tellers have done too many bad things and dare not look at others with their eyes, so they will look guilty.