You're filling out a few fields with fake data, but filling thousands of "fields" daily, for real. Those come from your behaviour: what sites you visit, when, with what frequency, what do you click, what do you install, who are your contacts, when do you sleep, when do you move, how often you send messages... I can not even imagine how long the list must be from all the things you can extract from someone's internet + mobile behaviour.

Edit: and that's what I find most troubling: that someone can know much more about me than myself. Things that you can only realize from looking at the data over time, things that are invisible when experienced on real time.