Does it not depend on context? For example, you don’t need to know anything about the user of a recipe website to know they are hungry and once they pick a recipe, some basic notion of what they are looking for.
On top of that, extremely course information can be gathered without tracking users. For example, you can probably get a country level location and maybe even language from the request the browser sends.
I can buy ads generically on a site, but what happens after the user clicks on the ad or just views the ad and then buys the product? How do you keep track of this when you can't measure the effect? The only way is to measure some form of before/after analysis of your conversions with or without the marketing campaigns... Too much noise to be able to do that on the web.
You don't need to know anything about a person to know that, at some point, they'll be hungry. So, if you're some company that happens to sell food nationally, you can just throw your ad at anybody. That's what I mean by "big generic brand".
However, if you sell, say, artisanal hot sauce, you'd pay more to target an affluent person that likes spicy food, rather than some random person that may be just be googling how to make macaroni and cheese.
On top of that, extremely course information can be gathered without tracking users. For example, you can probably get a country level location and maybe even language from the request the browser sends.