I would suggest that's because the advertising may be poorly matched to the content. Very likely nobody clicks on the ads and the only purpose they serve is keeping their product top of mind. I worked at a San Francisco based magazine many years ago. Their site received about a million visits a month. But virtually nobody was clicking on the ads for HP Computers. In my opinion, it was a poor match between site content and ad content. Bad effect on the human experience. No donut for you. :(
My approach has been to conceptualize the advertising blocks as blocks of content. The question to ask is: How do these blocks of content integrate with those blocks of content? An article about five hundred dollar GPS enabled shavers that take pictures underwater can be illustrated with a photo of the product. Of course you need a photo, it brightens up the article, everybody enjoys a picture of what you're discussing. You see it all the time in magazines from The Economist to People. It works, it's a perfect fit.
Now you link it with your affiliate code with a gentle suggestion in small type beneath the image, Available at WonderShavers.com.... 30% Off. The ad compliments the content.
The downside is that maintenance is higher than simply slapping up banners or adsense code. Product sells out so you have to keep an eye on that. Bummer. :( The upside is that my site visitors click the ads and buy stuff they are enthusiastic about, and they likely found the ads useful to their lives within that context. Good effect on the human exeperience. ;)-Y
It's not if your ads fit the content: It's about your content being relevant to the ads.
What it boils down to for me is, does the content complement the ads? Do the ads offer something the content is missing? Is the content appropriate for selling specific items? Or is the content too broad in scope?
An article about the planet Saturn is edifying and worthwhile reading. So is an article about how to select the best telescope to view Saturn.