Very true. The difficult problem in this case is that it's impossible to serve anything else, even though there are various techniques out there that still identify you when using private browsing (such as canvas fingerprinting -
https://securehomes.esat.kuleuven.be/~gacar/persistent/index...): even if they're using such techniques under the hood (and that study, apparently from 2014, says 5.5% of top 100k sites were using it), they can't use it for recommendations, for obvious reasons (browser vendors would get eaten alive, private browsing would be overhauled, this technique would no longer work, everyone goes home sad).