Because advertising does not require invasive and pervasive tracking.
The better question is: why are you so happy defending 24/7 surveillance of everything you do?
> Stasi or KGB weren't "opt in" for their citizens
What you're proposing isn't opt in either. You're proposing "pervasive and invasive tracking, or pay an arbitrary sum randomly defined by the tracking industry". That is, literally pay for access to internet without tracking, or else. All based on a completely unproven assumption that tracking is required for ads to function. It's not.
> while social networks definitely are.
Indeed. Because tracking is happening only on social networks.
> For example, Twitter won't allow me to access the images
Here's a direct link. And no, this pop up wasn't on an "opt out social network". It was on a random site (can't remember now which one): https://pbs.twimg.com/media/GTe23o5WwAACyNJ?format=png&name=...