As for cookies against ad networks, I browse with all third party cookies blocked. Some cookies from third parties are allowed. So that would not be the effective against people with high security settings.
And there is also the possibility of a brand new computer with no cookies. Then that kind of detection will not necessarily work for every one.
The owners of these websites don't care about false positives (like people on new computers). Their interest is in maximising the number of ads shown and nothing else.
And there is also the possibility of a brand new computer with no cookies. Then that kind of detection will not necessarily work for every one.