Back when Google started, the idea was to only show discrete text ads on search. Then they started showing image ads and created AdSense which basically shows Google ads on external pages. The real killer was when Google bought doubleclick. Until then Google didn't do ad tracking but with dclk (which has a tracking cookie) things went downhill fast. Ever wonder why Google ads domains are (or used to be) doubleclick.net? Because changing the domain would make Google lose all those yummy doubleclick tracking cookies.