Got this notification when I opened Chrome when coming back to my desk after lunch.
"We changed our privacy settings to allow us to snoop on what you're looking at and shove you ads accordingly. Feel free to opt out, but we'll probably opt you back in when you aren't paying attention."
A lot of times people will visit a website for a product they're interested in and may not immediately pull the trigger. When they see it later 3-4 more times, the chances of conversion are way higher.
Google probably doesn't really know if you purchased the product, and may not care, as you may want to purchase another.
I'm sure it's like gambling and microtransactions where the vast majority of income is derived from a small minority of people who aren't bothered by the onslaught of ads shoved down their throats.