The success rate of the algorithm is important. The average person doesn't want to see someone's else ads just because they're in the same network. No one wants to see their kid Minecraft ads.

IP are also constantly changing, at least once a month. For sure, they can't rely on one identifier, which is the IP address, because after it changes, all of the data is gone.

Also, you say things I didn't say. I said THE IP is not a really good identifier, but you also say I meant also other identifiers.

The IP can't survive on its own. The algorithm needs more than just one thing. It needs multiple things while if one is not applicable then we get another one.

An ad blocker already eliminates you to 40% of the internet users, which is a lot.