Simple. People use Google to locate things they want to buy. Ads that appear after a Google search for a particular product often result in sales. Google ads pay for themselves, so advertisers are willing to purchase, so Google makes money.
Visitors to the Fox website aren't going there to buy anything, they just want to read the news. Ads on a news site are annoyances, to be ignored. Social networking sites like Facebook have the same problem. People go there to meet friends and have conversations, not to purchase.
I find Amazon's "personal recommendation" ads interesting. They are targeted to specific users based on their past Amazon purchases. These days I actually want to see what Amazon thinks I should buy, because I often agree with them. I don't know how those ads are generated, but I bet they are very valuable to Amazon and its independent vendors.
Sometimes people are looking for something to buy. But I think more often people are searching for information, and with that search comes ads.
Visitors to the Fox website aren't going there to buy anything, they just want to read the news.
When I watch television or listen to radio, I get ads. I don't watch or listen because I want to buy something. I am interested in some TV or radio show. The ads fund the show. If nobody watches a show, no company or organization will buy ads.
I find Amazon's "personal recommendation" ads interesting. They are targeted to specific users based on their past Amazon purchases. These days I actually want to see what Amazon thinks I should buy, because I often agree with them. I don't know how those ads are generated, but I bet they are very valuable to Amazon and its independent vendors.
It's called personalization. Amazon keeps a database of every person that has an account. When you log in, the software searches your database, analyzes what you bought in the past, matches your buying pattern to similar products in another database, and they generates a web page customized to your past purchases. If they have other information about you, where you live, your age, sex, etc., this information is taken into consideration as well in selecting merchandise to show you.