Can you imagine if online surveillance happened in the real world?

You'd have people tailing you everywhere you went and it would be unbelievably creepy.

It would be made illegal immediately.

But on the internet, you can't see it, so we don't think about it that much.

Tailing you in real life (where you have to have a whole person tasked to follow you) just doesn't make business sense for advertisers… but it does make sense if they can do it with practically zero marginal cost.

Atoms have marginal costs that are many orders of magnitude larger than bits.