Use Hanlon's razor.

So, the scam here is from ad providers like google on anyone running ads. It's the reason that often you see ads for a product you just looked at forever: if you are shown an ad for a product, and then you buy that product, the ad provider gets a cut. If you click on the ad, that cut is higher. Now, clearly you already looked at the product without the ad, and at best it was a reminder of something you already wanted, but in the eyes of the contract, you bought that product because of the ad. That's WAY more of a sure thing than actually compiling a meaningful profile on the provider side. Now, on the opposite end, you DO have stories of storefronts sending out coupons, emails, direct to consumer ads, built on your viewing history on their site that ARE based on complex algorithms that know you better than yourself, which is how you get stories of the algorithm knowing people are pregnant before they do

I used to wonder this, too, and think that it couldn't be that effective. Then I went to my last job, and there were MULTIPLE people who said they actually liked the ads because they learned about new products that they would like. What's worse is that a couple of those multiple people actually clicked the ads and would buy things.

All that to say, yes, it is actually at least somewhat effective, and it erodes my faith in humanity.

The advertisers are paying for ads to he shown to people who have shown an interest in humidifiers. If they aren't clear enough that people who have recently purchased the humidifiers shouldn't be targeted, Google isn't gonna correct them and show the ads to fewer people.