And that's when things get seriously invasive and people start saying "Why are you using a 26kb tracking script?"
If you collect information about how often a user is moving their mouse and scrolling the page in order to tell how long someone has been on a page then you've moved from collecting what's useful to collecting everything you can just in case it's useful. That decision comes at the cost of invading everyone's privacy, and that needs to stop.
It's not like this data gives you an immediate and perfect readout of the user's entire brain, but it's not useless either.