They can be -- but not necessarily. It has a lot to do with conversions. Someone searching "car insurance" or "cheap flights" is most likely on the hunt for something they are willing to pay for; while someone searching "football scores" is most likely just looking for free information.
Of course this doesn't even scratch the surface of running a site for ad revenue that provides free information to common search questions, e.g., "how do I change the oil in my car"
Ads are actually more likely to provide the answer to your question, or the solution to your task, when you're searching for something to buy. Which may weigh into why Google puts so many of them on the page.
It's one of the things that prevents me from ad-blocking sites like Google. Some of the time I will actively be searching for the ads, and avoid the natural results, because the natural results for some product searches can be extremely spammy with "review" sites and comparison shopping sites with horrible user experiences dominating the results, while the paid results are usually decent retailers or distributors.
Of course this doesn't even scratch the surface of running a site for ad revenue that provides free information to common search questions, e.g., "how do I change the oil in my car"