Humans seek to explain and understand. We want answers and explanations. Our brains demand it. But our brains have developed many short-cuts to understanding and arriving at explanations. We call these biases. Many times our biases serve us well. Other times, they kill us. Bias is hard to overcome and it takes a lot of deliberate work and effort to do so.
Here is an excellent article from fivethirtyeight.com about the illusion of causality, explaining why our brains are primed to reach false conclusions. Go read it!