It's not impossible to do, but impossible to hide, and not feasible to do anyway. The technical complexity is massively higher than just using your browser his

It's not impossible to do, but impossible to hide, and not feasible to do anyway. The technical complexity is massively higher than just using your browser history, which means there would have to be a huge team that does it, and therefore no secrecy. Also for the same reason it would be massively more expensive than existing data collection, but the ads would sell for the same price,.so it's not economically viable.