We’ve gotten accustomed to banners that don’t interest us. And we’re accustomed to them having nice graphics. So we naturally associate the two things and think that a banner with nice graphics has nothing to tell us.
But we might look at a text-ad because we’ve grown accustomed to them being targeted at what we’re looking for. Basically these advertisements have marketed themselves into being looked at by any visitor.
This is a typical “no fluff, just stuff” situation. And in the end it means the visitors will see what they’re interested in, not some random cool graphics you throw at them.
As stated above, site visitors adopt this advertising mechanism, because it is efficient in leading them to pages of interest and can offer ton of things they really want to see. Someone coming across an article on adsense was probably looking for shoes or doing research on shoes when they came across the article.