In terms of tech, it's because what data you think they have on you is only a tiny fraction of what they actually have. I'm a backend developer who's worked on numerous web services and you'd be blown away by the amount of data companies collect. Think: full scanable replays of your browser session when visiting websites. At my previous job I could see literally every bit of identifiable data available for each user and could replay their browsing as an interactive video.

It's a massive invasion of privacy