But you don't notice all the ad spend that goes into making you want more stuff in general, or all the ads that make you feel a little uglier on a subconscious level
I find it hard to believe that return rates are high enough for this to be worth the trouble. It's much easier to believe that the advertisers are simply reacting to any signal for personalization, even if they received that signal too late.
> Maybe all that matters is that you might click the ad out of curiosity as you were recently looking into them
That serves the interests of Google or any other ad network; they don't really care about whether you eventually complete the purchase, as long as they get paid for your click. But the company actually selling vacuum cleaners should care.
This is incorrect. It is often relevant, and it is reflected in financial performance of personalized ads. Companies aren’t doing it for fun. They’re doing it because it is wildly profitable and that’s because people respond more to ads that are aware of that “past”. You may not have access to the “proof” but they absolutely do.