Long back when this started, OpenDNS was the largest public DNS provider. And I know at one point they started "hijacking" google requests - you can read this here : http://www.dslreports.com/forum/r21284386-Google-Com-Hijacke.... This was in 2008. They used to also hijack DNS requests to show their own ads. And i am pretty sure many ISPs always played around with DNS in this way.

In 2009, Google launched its own Public DNS - the reasons given were to make the web faster and safer, and i think there is some sense in which they have delivered. But I think another reason was defensive - provide an alternative to OpenDNS and to crappy ISP DNSs.

In 2014 - OpenDNS stopped using DNS to show ads : https://umbrella.cisco.com/blog/2014/05/29/no-more-ads/.

See my reply here: https://news.ycombinator.com/item?id=16733619

tl;dr: Google succeeds when people use websearch. Cloudfare succeeds when people use the Internet in general. People use websearch more when the Internet is fast and reliable. ISPs resolvers are frequently not fast nor reliable. Thus if you can improve peoples Internet experience with a better resolver, you improve your core business.

No need to track anyone via DNS for this to be successful.