When they track half of humanity, they can see all kinds of patterns that they, and other people can exploit.

Let's turn your question around. If all this information is that useless and harmless why are all corporations and governments spending tens of billions of dollars acquiring and keeping?

Personally, I have two reasons to avoid Google and other tech monopolies: 1. I don't want the government to be able to subpoena my data 2. I don't want to be cut off from my digital life just because some employee clicks a button. That means I'm self hosting as much as possible.

This often reflects and amplifies societal biases. For example, users that Google identifies as female are less likely to be shown ads for high paying jobs [0]. This creates a feedback loop: if a certain user demographic is less likely to be shown a given result, they are less likely to view it, which in turn makes them even less likely to be shown it in the first place.

[0] https://spectrum.ieee.org/amp/women-less-likely-to-be-shown-...

From your comment, it seems like your main concern is that governments can access your data. But what if beyond&above those governments, there is a single entity that's far more powerful? Would you be willingly hand the data to them? Because that is what Google is. Or is becoming. Or, if you're very sceptical: can easily become.