People find ways around it, for example for a while Flash didn't respect incognito so ad networks just used their cookies instead (I think at one point html5 storage was being used too, but could be wrong).

There's been various HN posts about how people circumvent browser privacy settings to track you across websites and even tricks such as generating an almost unique key by hashing together browser, fonts installed, IP address, etc.. I'm not sure of the current state of play in that war though.

I guess if you really don't want people to track private searches with your accounts, use a whole other browser where you're not logged into Google in incognito mode.

not if you are logged in using your google account