I know people who run good websites do it because they love what they do, but let's face it, now that people can earn good money through various ad programs (especially adsense for most people as its the easiest and most convenient), they also do it to get a return on their time and investment, and for some extra income which always comes in handy. Of course, some people even make a living off their sites.
What if you suddenly found your income was down by 70-80 percent because of poor adsense performance and this figure was not getting back for months to prior levels, would you still have the same desire to produce quality content or run the sites at all?
My personal view is that I am not sure if I would be able to devote time and effort and expenditure to run sites if I wasn't getting a return. I know back in the early 2000s a lot of people created content without expecting anything in return, but as far as I am concerned, the only people who might do that these days are people running personal blogs or some non-profit oriented organisations.
Now that you are expected to earn something in return, the only way to stay motivated is to actually keep seeing income roll in.