Because there's a tradeoff between 'privacy and security' and 'convenience', and most people like to fall somewhere in the middle.

It's like locking your doors on your house. Sure, a burglar could break the door down, smash a window, follow you around and steal your keys, etc., but it adds a level of security and precaution that keeps some of the bad guys out while not being insanely inconvenient.

I care about the privacy and security of my home, but I'm not going to live in a bunker.