You are correct, and if that were the end of it, people like myself who are concerned about it could rightly be labeled paranoid.
However, the collected data create an intimate, detailed portrait of you. Data aggregators can know how old you are, your general state of health, where you have been, your interests, including sexual interests, your political views both expressed and unexpressed, your associations, your associates,
your family, your daily habits, your unusual habits, your credit score, whether that credit score is accurate, etc, etc.
And, this is true for everyone you come into contact with: friends, family, business associates.
An AI could, with this data, model you and your behavior more accurately than could your own spouse.
Moreover, not only must you trust the people with access to that data, and everyone they sell it to, to only ever use it for targeted ads, but you must trust everyone who will ever have access to it, forever.
While you may believe you're boring and your views are entirely uncontroversial, times change. Or, hell, with enough data and motivation, someone can paint a damning portrait of you made up whole cloth from true, innocuous details.
You have to just hope that neither you nor anyone you associate with become a political target, or the target of criminal activity, or garner just plain negative attention, for whatever reason, because if so, you and your loved ones will be trivially easy to find and manipulate.