Problem are not ads being relevant. Real problem is much more sinister. It's about being able to influence your behavior thanks to knowing your habits. Shill new running shoes from a different perspective so you find it more palatable or easier to influence you to buy something you don't want. Slowly these patterns emerge and can be exploited. In some cases they can start to reveal secrets you don't want revealed, like that case where Target sent pregnancy ads and coupons to teenagers and causing bunch of issues with their parents. It gets worse if they start selling your data to others. Imagine a politician making targeted ads towards select group of people based on their preferences telling them what they want to hear in order to get elected. Data can easily be abused in such a way.

You say it doesn't work on you, which I doubt but even if that is so, on majority of people it will work. Privacy must be taken absolutely and seriously even though you have nothing to hide. Just like freedom of speech must remain even though you have nothing to publicly say.