salam guys,
These astrologers don't know the future they just do predictions about you in a cunning way. They can have some knowledge about you, not about your future, but your present status or past experiences somehow.
For example if someone had a 90 year old grandpa/grandma and a 13 year old brother, they would make a guess and say, "this young man will not be married unless your grandpa is dead."
If you knew a person with a fatal disease, with a little knowledge of his/her condition, just like a doctor you could say "he/she will die in 5 years!" That's not a big thing. All you can do is guess, make up scenarios !
If you saw cumulonimbus clouds all over the sky you can pretty much predict what's gonna happen
What these fortune tellers do can easily affect people psychologically. It was 2 or 3 years ago, in summer time my mom and two aunts came to visit me for a one-week holiday. While we were walking and shopping in a touristic place, a fortune teller approached us, took my older aunt's hand, turned it over and began to examine her palm. Then she said that a woman in my uncle's office was too jealous of their marriage ! My aunt got really suspicious after that, she was annoyed out of nothing! For two days she thought about their marriage, didn't enjoy her holiday at all. I'm saying... Out of absolutely nothing, she got stressed out and spent her holiday with doubts ! Just because of that fortune teller lady.
This can ruin marriages, destroy lives and make you sick. Because it directly affects your mood, your brain. I can understand why it is haraam!
So seek refuge in Allah from the accursed shaitan. Trust in Allah. May He protect us all from all kinds of evil.
Salam.