We live in the information age and information now is the ultimate power. Many people don't believe it, the same way that before the Civil and Crimean Wars not everybody believed that industry is the ultimate power of the industrial age.

By letting Google collect enormous amounts of data about you, you are letting them have power over you. It can be used for pushing you to buy stuff against your rational will or for killing you, if say at any point you decide to not fully cooperate with a future totalitarian government.