Why people complain of not earning enough money with AdSense

I hear very often about people complaining they don’t make enough money with AdSense. Let’s look closer what happens actually.

If you have a small website well placed into the Google search results pages, it is obvious that there are a lot of people just looking around, which actually just click on your AdSense ads, but are not wiling to purchase anything from these destination websites. Say that for example maximum density of your keywords could be linked to at most 50 different websites or profiles of websites. The advertisers can control how they spend their budget, so that from 50 ads displayed into your website, 30 of them also bid for other keywords, and their higher bids are for ads NOT displayed into your website because of your content.