Not sarcasm - I think that while it's obviously a different scale, the reasons are similar and boil down to "we don't want that as a society" and "the environment this creates is not conductive to a free and informed rational decision". Many people don't understand the value of their data and the risk it poses, there is an information imbalance, there is a power imbalance (the company sets the terms, and you only get to take it or leave it).

The pre-GDPR situation also showed that the market doesn't really work, because everyone was collecting your data, people have limited energy and incentive to care because it doesn't cause immediately visible pain. It's similar to workplace safety - we don't allow employers to create easily avoidable dangerous situation in exchange for extra pay either, for similar reasons.

Most importantly, data grabbing is not necessary for advertising, it's just slightly more profitable and thus everyone does it, eventually pushing the "good" (privacy-friendly) players out of the market. If we want to change that, we need a de-facto ban (which a properly implemented GDPR would be, because so many people will click "No" if given a truly free choice that showing the popup won't be worth it).