Mostly, because they were BORN rich.
(Although anyone can improve their chances of getting rich.)
Think about the rich people you know… Did ALL of them achieve their fortune by themselves? No. But even if they did it all by themselves, their CHILDREN will be rich without lifting a finger. So, simple mathematics say that at least 50% of all rich people were simply born rich. (Thus the expression “born with a golden spoon in their mouth”).