The more you scroll, the more ads they can serve on one page. So if you scroll to the bottom, don’t see the results you want, you’re likely to try to reword what you were searching for which will bring up new results and more ads. When you think about the fact that 4-5 of the first results are ads generally (if not more) and you have to scroll past those to get a result that isn’t an ad, you recognize that they are maximizing time spent looking at ads because that’s what they are selling to their real customers (the ad services for whom they aggregate).

This scenario makes it more likely that you will click on a sponsored result, backtrack, scroll some more, not see what you’re looking for, re-word your search query, click on maybe another sponsored result, backtrack etc.