Because those websites don't contain ads provided by the Google network, while Google can choose to send the user to websites that contain ads for which they ar

Because those websites don't contain ads provided by the Google network, while Google can choose to send the user to websites that contain ads for which they are going to take a cut. Obviously the best for Google would be to send the user to a website with the best content and that contains Google ads but that's often not both the case. It's an optimization problem, and generally you cannot optimize for two variables, profits and user satisfaction. Maximizing only for profits would probably lead to completely garbage results so they probably have to compromise.