I wouldn't be surprised if the actual reason was they are capturing extra sales from people who bought something, liked it a lot, and then bought another to give to their kids or for their second home. My mom buys me random chotchkies all the time after she bought them and liked them, and if my parents were well off I could easily see her buying me a vacuum or a new mattress after she got herself one.

For 99% of people that probably doesn't happen, but the people with the money to do that probably spend a TON of money, so they are worth targeting with ads.

So they are misattributing the additional sale to their suggestions instead of the product being an advertisement for itself.

I suspect adtech industry would shrink down half its size if it could accurately attribute sales to ads. It sometimes smells as if it was standard practice to convince the client (intentionally, or unintentionally by also convincing yourself) that correlation is causation and therefore their success is attributed to ad spending.

The working vacuum in your house is a significantly better ad than whatever amazon shoves down your throat.