Imagine if someone was following you around with a clipboard writing down everything you do, then rifling through your bookshelf to make note of certain books on the bookshelf, and then using that to target ads at you.

You'd say that's a ridiculous and illegal thing to do without you explicit consent, right?

Maybe you personally don't mind and would be happy to offer that consent. But they're doing it without your consent, regardless of whether you want it or not.