The main reason for this is that insurance companies are looking for ways to devalue or deny your claim. Social media is one of the leading places that insurance companies search for details you may discuss publicly. Specifically, they look for any posts or comments you make related to the crash, your injuries or your current state of health.
While this may surprise you, it is a good idea to remember that insurance companies are not on your side. However friendly their employees are trained to sound on the phone, they are, first and foremost, for-profit businesses. Their goal is to protect and sustain profit for the company. Therefore, they will actively look for ways to pay out as little as possible.