There are radio stations, financed through ads. And they check if people are listening by calling random persons to ask them what station they are listening to.
So this is a viable business model and nobody is stopping anybody from putting plain pictures and links on sites and just estimate the page visits, but online advertisers want to know more. They always want more.
At the same time, a browser is the essential software to browse the web. So this is as if your TV was like:
Yo, many people mute their TV during commercials and don't pay attention, which kills the poor networks. So I made a deal with advertisers and will check what your doing, while I provide unmutable ads , but don't worry, your privacy is very important to us and we only care about providing to you the best TV experience possible.