Why do brands keep pushing traffic to the Facebook/Twitter pages instead of their own pages? They should create dynamic interesting content on their main pages

Why do brands keep pushing traffic to the Facebook/Twitter pages instead of their own pages? They should create dynamic interesting content on their main pages and use social media to push traffic towards it instead of using social media as a goal. Ownership of your following is really important and at the end of the day the only way to do that is to have a proper standalone website.